Maha-Vishnu is a plenary emanation of Lord Krishna. All the Brahmas that govern the various galaxies live only for the duration of one of his exhalations, after they have emerged from the pores of his spiritual body.
Living beings are automatically reabsorbed into the body of Maha-Vishnu at the end of Brahma's hundred-year lifespan. But thus absorbed into Him, the individual beings distinct from Krishna, God, retain their own identity, and as soon as, by the Lord's will, creation returns to the manifested state, all beings, hitherto inactive and as if asleep, find themselves free to resume their various activities, in accordance with their past conditions of existence. This is what is called the principle of awakening after sleep and resuming one's own activities. When a person sleeps at night, they forget their identity, their duties, and all their activities in the waking state. But as soon as they regain consciousness, everything they have to do comes back to them, and they resume their activities. Similarly, living beings, as spiritual beings, remain in the body of Maha-Vishnu for as long as the annihilation of the galaxy lasts, but as soon as the time of creation arrives again, they awaken and resume their unfinished tasks.
Lord Krishna explains this: With the day of Brahma, all kinds of beings are born; and when his night comes, all are annihilated. Endlessly, day after day, the day is reborn, and each time, myriads of beings are brought back into existence. Endlessly, night after night, night falls, and with it, all beings are annihilated, powerless to do anything about it. There is, however, another world, eternal, beyond the two states, manifested and unmanifested, of matter. Supreme world, which never perishes; when everything in the material galaxy is dissolved, it remains intact.
At the end of an age, when the lifespan of Brahma comes to an end, all material creations return to Me, and at the beginning of the next age, when the time is again favorable for creation, through My external power, I create anew.
Material nature acts under My direction; under My direction it brings forth all beings, both moving and immobile. By My command, it is created and then destroyed, in an endless cycle.
The fact that the Lord existed before the activation of the creative energy of the three attributes and modes of influence of material nature shows that He was not produced by material energy. His body is entirely spiritual and in no way differs from His Person, which is not the case for conditioned souls. Before creation, the Lord was in His kingdom, One and Absolute.