the first thing to understand is that the "flying pancake" model you see when you google "flat earth" is nonsense, that's put out there to make flat-earthers look stupid

the actual theory is that the Earth is basically the BEDROCK of the universe, so there is no space under it. the furthest humans have dug into the earth is 8 miles, and it's just rock, rock, and more rock. so that's basically all we know - again we can use Minecraft as an analogy here- when you go deep enough, it's just pure solid bedrock. if there's more than that, we haven't found it yet.

some models, like in the attached pic, theorize that you may eventually run into water again. we don't know, because we've never dug that far. one of many fascinating mysteries to ponder!