That’s a good question.

I haven’t seen any farms or mines or anything else like that here. And as for fish, well, there is an ocean but since the locals all hate it, I highly doubt any of them go fishing in it, so the fish people eat here can’t possibly from there.

If I had to guess, they must get their food and everything else they require to sustain themselves from the sort of technology that brought us all here, somehow. After all, if they can transport people from universe to universe, they surely can transport other things too and do many other things that are beyond what the people in our universes are capable of.