Garbhodakashayi Visnu's inner material universe is surrounded by and encased in a greater material universe called a Brahmanda.

All Brahmanda universes originate from Maha-Visnu along with unlimited others, as seen in painting below.

All of Maha-Visnu's Brahmanda universes originate from His breathing and the pores of His skin and populate 1/4 of the Spiritual Sky called the temporary material creation. 

3/4 of the Spiritual Sky has no decaying material mundane Brahmandas but has eternal spiritual planets called Vaikuntha Planets and the central Goloka Vrindavana abode of Radha and Kṛṣṇa, where no old age, disease or death takes place.

Deep inside, each massive Brahmanda universe created by Maha-Visnu is a "secondary universe" where He expands as Garbhodakashayi Visnu.

Each Brahmanda universe is quadrillions of miles in diameter. 

From Garbhodakasayi Visnu inside His material univers inside each Brahmanda, a Lord Brahma appears from Garbhodakashayi Visnu and builds the planetary systems in His particular inner material universe.

Our material universe inside our Brahmanda is only 4 billion miles in diameter according to Srimad Bhagavatam; however, other universes deep inside their Brahmandas are many, many different bigger.

This also means there is only one Maha-Visnu in the material creation; however, there are billions of Garbhodakashayi Visnus and Lord Brahmas.

Both Garbhodakashayi Visnu and Brahma are within their own "secondary universe," of which there are billions of them as explained above.

As said above, each secondary universe is surrounded by and encased in a massive outer universe shell known as a Brahmanda.*.