There's a flashback scene apparently at the beginning that shows Amazons, Atlanteans and Humans after repelling Darkseids first attempt at invading earth like 10k years ago and spreading the mother boxes between them, which was apparently confirmed by the first trailer.
The pic just released is most likely in Batman's flying fox or the underground tunnels where they fight parademons.
I don't remember but there are more, the only thing that's not clear is Superman's return, IIRC there are a couple versions, the one I remember is Parademons stealing the body, Batman intercepting it and Cyborg using Apokolyps tech to revive him, there's a resurrection meta scene with Pa Kent kind of like Death of Superman comic but in reverse, where Pa Kent sends him back to the living (I remembered this one because I called it
) but it's still one of different versions going around, and he joins in in the middle of Steppenwolfs final bos fight.
Apparently they don't kill him but send him back through a boom tube where we get to see Darkseid (or his feet) receive him back after his failure