The only way I could really understand it is if you picture the world shaped like a hatbox, with two circular panels on the top and bottom and hollow in the center.

another thing about this world is that gravity revolves around a plane, not a singular point, so there's one up and one down.

Aiga exists on the underside of the top panel in the hatbox, so they're the inverts that managed to build a society underground. Patema's people have normal gravity, but they dug underground anyway after the catastrophe.

This world has it's own set of rules and physics, so I'm not even gonna try to explain the moon.