Krsna, as His original two-armed Form, never leaves Goloka-Vrindavana.

The Krsna that does come to the material universe is actually a Visnu-tattva expansion from the original two armed Krsna, who plays the part of the original childhood Krsna and enters the material creation.

Srila Prabhupada - "Kṛṣṇa as His original childhood form NEVER leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His Viṣṇu-tattva expansions."(BG, As It Is Ch 10 text 37)

Srila Prabhupada - "The original Lord Kṛṣṇa never leaves Goloka Vṛndāvana. All the plenary expansions are one and the same Viṣṇu-tattva, and there is no difference in Their potency." (SB,  Canto 3 Ch 1 Text 34)

Only in the facsimile version of the original Vrndavana does Visnu, as the form of Krsna, kills demons.  

In Goloka-Vrindavana with the original Krsna, there are no demons, not even rumours of demons, because Vaikuntha means one is free from all anxiety.

Only in the material creation do demons exist who are killed not by Krsna directly, but by His Visnu-tattva expansion playing the part of the original Krsna in the material creation.

Krsna is the original source of all Narayana or Visnu forms, who are all His expansions residing in the quadrillions of Vaikuntha Planets that surround His central abode of Goloka Vrindavana. 

Krsna further expands via His first Visnu-tattva expansion, Balarama, as eventually Maha-Visnu (Karanodakshayi Visnu), who creates and manages the unlimited material universes called Brahmandas.

As said above, Krsna is the Supreme Personality of Godhead and cause of all causes, who resides in His personal abode of Goloka-Vrindavan and never leaves.

However, Krsna is also the original source of all Narayana or Visnu forms of God, who are all His expansions that reside in the quadrillions of Vaikuntha planets that surround His central abode of Goloka-Vrindavana.

In actual fact, the Krsna who comes to the material creation as His original Form is a Visnu-tattva expansion of the Krsna in Goloka-Vrndavana.  

The Visnu-tattva 4 armed expansion of Krsna manifests as a facsimile of the original 2 armed Krsna. 

This Visnu expansion then enters the material creation and plays the part of the original Krsna in Goloka-Vrndavana.

In other words, the Krsna who enters the material manifestation is really a Visnu 4 armed form of the Lord.

This Visnu expansion of Krsna comes to the material creation and plays the role of the original two armed Krsna. 

Srila Prabhupada - "Because Kṛṣṇa never leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His (Visnu-tattva) expansions." (BG, As It Is Ch 10 Text 37)

Srila Prabhupada - "The original Lord Kṛṣṇa never leaves Goloka Vṛndāvana. All the plenary expansions are one and the same Viṣṇu-tattva, and there is no difference in Their potency." (SB Canto 3 Ch 1 Text 34)

Srila Prabhupada - "The biggest and highest planet in the brahmajyoti is Kṛṣṇaloka, or Goloka-Vṛndāvana, where the Supreme Personality of Godhead, Śrī Kṛṣṇa Himself, resides. Lord Śrī Kṛṣṇa NEVER leaves Kṛṣṇaloka." (Sri Isopanisad 12)

Srila Prabhupada - "Kṛṣṇa is the original Supreme Personality of Godhead, and Baladeva (Balarama) is Kṛṣṇa's immediate expansion. Both Lord Kṛṣṇa and Baladeva appeared as sons of Vasudeva, so both of Them may be called Vāsudeva. From another point of view, because Kṛṣṇa NEVER leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His expansions." (BG, As It Is Ch 10 Text 37)

Srila Prabhupada - "Ādi caturbhuja, the original expansions from Baladeva (Balarama) are Vāsudeva, Saṅkarṣaṇa, Pradyumna, and Aniruddha. All of Them are Viṣṇu-tattvas, or nondifferent Personalities of Godhead. In the incarnation of Śrī Rāma, all these different expansions appeared for particular pastimes. Lord Rāma is the original Vāsudeva, and His brothers were Saṅkarṣaṇa, Pradyumna, and Aniruddha.

Aniruddha is also the cause of Mahā-Viṣṇu, from whose breathing the Ṛg Veda appeared. All this is nicely explained in the Mārkaṇḍeya Purāṇa. In the incarnation of Lord Kṛṣṇa, Aniruddha appeared as the son of the Lord. The original Lord Kṛṣṇa NEVER leaves Goloka Vṛndāvana. All the plenary expansions are one and the same Viṣṇu-tattva, and there is no difference in Their potency." (BG, As It Is Ch 10 Text 37)

From the original Krsna, who is always in Goloka-Vrindavana and never leaves for the material creation, also expands as His Visnu-tattva expansions for entering the Vaikuntha Planets.

As explained above, Krsna as His original form, expands to His Visnu-tattva who then playing the part of the two-armed Krsna in the material creation.

For the creation of the material worlds, Krsna enters the material creation via His first Visnu-tattva expansion, Balarama, who is Maha-Visnu (Karanodakshayi Visnu)

Maha-Visnu creates and manages the unlimited material massive universes called Brahmandas.

These unlimited number of Brahmandas, coming from the Body of Maha-Visnu's pores and breathing, house within them individual material universes made up of material qualities (both subtle material energy and gross material elements).

In other words all Brahmanda universes have a secondary material universe deep within them, this means each massive Brahmanda material universe covers, encases, and surrounds an inner smaller secondary material universe deep inside with 7 outer layers of material qualities that make up the Brahmanda.

Maha-Visnu again expands again as Garbhodakashayi Visnu, who creates this secondary material universe deep inside the Brahmanda.

Then from Garbhodakashayi Visnu, Brahma manifests who creates the different planetary systems within each inner material universe that spread out as a flat looking disc made up of many spherical planets that spread out as individual global planets to nearly the edge of this inner single material universe, as explained in Srimad Bhagavatam.

Because of the massive solid material elements of the Brahmanda, no one inside the inner material universe can see outside of their own inner universe. Srimad Bhagavatam explains.

This means from within our small material universe inside our Brahmanda, we cannot see the billions of other Brahmanda's around it.

Srila Prabhupāda - "Each material universe (inside their Brahmanda) is covered by various layers of material elements, and therefore, although the Brahmanda universes are clustered together; we cannot see from one universe to another. In other words, whatever we see is within this one universe." (SB, Canto 5 Ch 21 Text 11, Purport)

Srila Prabhupada - "There are innumerable universes, and each universe contains one Brahmā, one Śiva, one sun, one moon, one Indra, like that, officers just like. So there are innumerable universes, so innumerable Brahmā, innumerable Rudra, innumerable suns, innumerable moons, everything innumerable. But each brahmāṇḍa contains one each. So, how does it happen? It is explained in the Śrīmad-Bhāgavatam. So Lord Caitanya is reciting that story.

eka dina dvārakāte kṛṣṇa dekhibāre brahmā āilā—dvāra-pāla jānāila kṛṣṇere

One day, when Kṛṣṇa was king at Dvārakā, with sixteen thousand queens, so one day Brahmā of this universe came to see Him. Brahmā āilā—dvāra-pāla jānāila kṛṣṇere. And as... The same etiquette was there also. This is also coming by paramparā." (Lecture on CC Madhya-lila 21.49-61 - New York City, Jan 5, 1967)

Girirāja - "Right now Kṛṣṇa is in so many different universes."

Srila Prabhupāda - "Yes."

Girirāja - "And He is having the same pastimes."

Prabhupāda - "Yes."

Girirāja - "And in each universe, there is an Arjuna."

Srila Prabhupada - "Everything."

Girirāja - "Everything. So is it the same living soul, Arjuna, in each universe, or a different living being may be taking that position?"

Srila Prabhupāda - "Take it for granted, different. What is wrong there? After all, everyone is Kṛṣṇa's expansion. Ānanda cinmāyā-rasa-pratibhāvitābhiḥ (Bs. 5.37). We are also expansion. (aside:) Hare Kṛṣṇa. But still, we have got individuality. Kṛṣṇa proved it—I explained that in Vṛndāvana when everything was stolen by Brahmā. Again, another batch of cows and calves and cowherds boys. (aside:) Hare Kṛṣṇa. Immediately. What is the difficulty for Kṛṣṇa? Is it clear or not? You wanted to clarify. Is it clear or not?" (Morning Walk - April 10, 1974, Bombay)

In other words because we are all expansions of Krsna, it is possible in each universe that there is a different Arjuna, why can't there be a different jiva-soul in each universe playing that role of Arjuna?

Deep inside each Brahmanda, each of their Brahmas create the different planetary systems.

Our universe inside our Brahmanda, Brahma, builds 14 planetary systems.

Each inner material universe within their Brahmanda has a different number of Planetary Systems according to their size.

Karanodakshayi Visnu (Maha-Visnu) is actually sleeping and is always dreaming the activities within each of the secondary material universes deep within the unlimited number of Brahmandas.

Those dreams Maha-Visnu have are all the activities happening within each of the Garbhodakashayi Visnu's secondary universes.

In painting below, Karanodakshayi Visnu (Maha-Visnu) is seen laying down sleeping, and Garbhodakashayi Visnu is seen within each Brahmanda.

When Maha-Visnu dreams, He first creates the entire material universe known as the Maha-tattva (matter), which takes up 25% of the spiritual sky or Brahmajyoti.

The other 75% is the active personal spiritual world (anti-matter) that is ever fresh, always youthful and never ending, known as the perpetual Vaikuntha planets of Visnu and Krsna's central abode of Goloka-Vrindavan.

Within Maha-Visnu's 25% of the spiritual sky called the temporary material creation, we find it is always in a state of decay, decline, and impermanence.

It is where jiva-souls who have chosen to enter the material creation, are forced into repeated birth and death because of the temporary nature of the material creation, forced to possess (or take) temporary material bodily vessels or containers.

The visiting jiva-souls in the material creation are forced to take repeated births again and again because the material bodies they are in eventually wear out, get old, break down, and cease to operate

To make it clear,

1 - Within this temporary material creation, we find unlimited individual massive single Brahmanda universes coming from the Body of Maha-Visnu.

2 - And deep inside each of those massive Brahmanda universes, there is a secondary smaller universe where Maha-Visnu expands Himself as Garbhodakashayi Visnu.

3 - And from Garbhodakashayi Visnu, Lord Brahma appears from His naval at the top of a lotus flower.

4 - Then Lord Brahma builds the many planetary systems inside each of the inner secondary universes deep inside their individual Brahmanda.

There is only one Maha-Visnu in the material creation yet an unlimited number of Garbhodakashayi Visnus and Lord Brahmas.

Also, it is important to understand the amount of planetary systems deep inside each Brahmanda, vary in size, some larger, and some smaller than others.

Our material universe within our Brahmanda has 14 planetary systems and is considered a small universe explained in Bhagavatam as 4 billion miles in diameter.

Some Vedic cosmologists think this figure of 4 billion mile's has not in diameter describing our material universe inside our Brahmanda is too small of a figure for the real size of our small material universe.

Some Vedic scholars suggest the figure of 4 billion miles or 500,000,000 yojanas, as decribed in the 5th Canto of Srimad Bhagavatam, is really a code that needs decoding because they claim just our Solar System is said to be 24 billion miles in diameter.

Another answer is that measurement can be different on the different planetary systems, just like the fabric of space-time measurement is also different on all planetary systems.

For example, 100 years on the Brahmaloka planet on the highest planetary system is 311 trillion and 40 billion human years long.

So, there may be far more to this figure claiming our material universe is just 4 billion miles in diameter.

Other universes inside their Brahmanda have many more planetary systems than just 14 and are therefore much bigger than 4 billion miles in diameter.

The fact is, the size of each Brahmanda and their inner universe depends on how many heads Lord Brahma has.

Some Lord Brahmas, like ours, has only 4 heads, while other Brahma's within their universe inside their Brahmanda, may have 10, 20, 50, 500, 1000, 100,000 or even a million or more heads.

In this way, all the quadrillions of Brahmanda universes coming from the Body of the sleeping Maha-Visnu (as seen in painting below), all have a secondary inner universe deep inside it also seen in painting.

Each massive Brahmanda universe has 7 material coverings or layers that surround and encase each of the inner secondary material universe.

This means the massive Brahmanda surrounding its inner secondary universe within is 44 quadrillion 444 trillion 444 billion miles in diameter or bigger.

The full size of the Brahmanda, if that measurement above is from the outer edge of the inner secondary universe, is actually twice that size.

The sleeping Maha-Visnu (Karanadakashayi Visnu) expands Himself into the inner secondary universe deep inside each Brahmanda as Garbhodakashayi Visnu.

This secondary material universe is in the centre of each massive Brahmanda universe.

In the Srimad Bhagavatam, Bhu-Mandala is the earth mandala (not to be confused with our small earth globe); it is a disk 500 million yojanas in diameter (4 billion miles in diameter)

One yojanas is equal to 8 miles in diameter, 500 million yojanas is 4 billion miles in diameter 

The word Earth has many explanations.

For example, our small Earth global round sphere is only 24.901 miles in circumference and is 7,917.5 miles in diameter; it is NOT Bhu-Mandala (our universe)

Bhu-Mandala is 500 million yojanas in diameter (4 billion miles in diameter or 12.6 billion miles in circumference).

A yojana is a unit of distance about 8 miles long, and so the diameter of Bhu-Mandala is about 4 billion miles, which is the size of our single material universe.

Our Earth is not flat! It is a round global sphere.

Srila Prabhupada - "On the ground, our earth looks flat, but when you go up, you will see our Earth planet is a ball." (Lecture New York City, Dec 1966)

The Sanskrit word dvīpa has two meanings.

1 - Islands or continents on a planet.

2 - Globes or spheres in outer space.

Bhu-Mandala is marked by qqqcircular features designated as islands (dvipas that can also mean planets) and oceans.

Srila Prabhupada -"The planets are called dvīpas. Outer space is like an ocean of air. Just as there are islands in the watery ocean, these planets in the ocean of space are called dvīpas, or islands in outer space." (CC, Madhya 20.218, Purport)

Therefore, Bhu-Mandala is far too big to be our small earth globe.

But, this does describe our small single universe with its 14 planetary systems deep within our surrounding Brahmanda.

Therefore, the 14 planetary systems deep within our surrounding Brahmanda would fit into the circumference of Bhu-Mandala (4 billion miles (500,000,000 Yojanas diameter) our universe.

The jiva-soul can stay for an almost eternity in the temporary decaying material universes cycles of annihilation and creation if they choose.

When Lord Brahma dies after 100 of his years (311 trillion and 40 billion human years), there is an equal period of his life span of 311 trillion, 40 billion human years where nothing living exists, everything in the material universe remains dormant for that enormous amount of time in the Body of Maha-Visnu.

Then, the next material creation of Brahmanda's manifest from Maha-Visnu (Karanadakashayi Visnu)

All jiva-souls have merged as individual units into the Body of Karanadakashayi Visnu (Maha-Visnu) and remain that way until the next creation of Brahmandas when they again manifest from Maha-Visnu's spiritual body as seen in painting below.

In other words, in the next new creation of Brahmandas coming from the Body of Maha-Visnu, all the dormant jiva-souls awaken and continue on again within a secondary universe that is deep inside each Brahmanda outer universe, from where they left off in the previous inner material universe.

As explained, a secondary smaller universe is deep within each of the massive surrounding Brahmanda universes.

Our Brahma lives for 311 trillion and 40 billion human years (100-year life span of Lord Brahma)

It is important to understand at the annihilation of each Brahmanda universe and its inner secondary universe, Garbhodakashayi Visnu temporarily merges back to be one with Maha-Visnu until the next Brahmanda universes are manifest from the breathing and pores of Maha-Visnu.

Then the process repeats itself, the sleeping Maha-Visnu (Karanadakashayi Visnu) expands Himself into the inner secondary universe that is deep inside each Brahmanda as Garbhodakashayi Visnu that is in the centre of each Brahmanda universe.

The jiva-souls are suspended in inactivity (dormant) within the Body of Karanadakashayi Visnu (Maha-Visnu) for 311 trillion 40 billion human years.

The jiva-souls re-emerge and continue on (their karma) from where they left off in the previous creation..*^*..