When a world system has gone through devolution and is in statis, before the next evolution, where are all the beings from that world system? I've read some of them are born into brahman realms, but surely not all of the trillions of different beings alive on earth are born there during this time? So what happens to them while there are no lower realms to be born into?
During the destruction of the world, most living beings become Brahmas and dwell in Brahma realms which are not affected by the fire. One hundred thousand years prior to the end of the world, a type of Devas called Loka Viyuha, who foresee this doom, come down to human world disguised as humans, announce the coming obliteration. Most beings on hearing this warning get rid of their reckless way of life and practice meritorious deeds and are born in Deva realms. From there they meditate and attain Jhana to get to Brahma realms. Hell beings are also born in human realm due to their past good Kamma and go up from there. Therefore all beings except those who profess wrong views (Nihilism AKA niyata miccha ditthi) reach Brahma loka at least once.
Those who cannot escape the destruction are born in a hell called Lokantharika Naraka.
The Lokantarika Hell
When three universes meet, there is space where Lokantarika hell exists, where neither sunray nor moonbeam can reach. Complete darkness reigns there. Beings in this hell cling to the walls of the universe in complete darkness. They mistake the other sinner to be some food, and try to bite each other. On doing so, they fall down into the icy/acidic water and perish.
Ignorance in beings creates universes. Universes create suffering in beings. Suffering in beings creates evolution in universes. Evolution in universes creates knowledge in beings. Knowledge in beings creates karma.
Positive karma leads to the end of ignorance in beings. The end of ignorance in beings leads to the end of suffering in beings. The end of suffering in beings leads to awakening of beings. Awakening of beings leads to final liberation of beings. Final liberation of beings leads to deathless births of beings outside universes.
Deathless births of beings outside universes and karma of beings inside universes creates destruction of universes. When destruction of universes overweights creation of universes, devolution arises. When devolution arises, the bodies of beings in universes are destroyed. When bodies of beings in universes are destroyed, their karma remains.
Remained karma creates ignorant beings. New ignorant beings come to universes from higher/lower planes. Ignorance in beings creates universes...
The cycle is repeated.