Things that fall down are more dense than air. An apple is more dense than air therefore it will settle below the air. Nothing else changes as to why things work. Just the way you look at it. Why do different liquids stack on each other? Density. If you had a tube tall enough you would witness water “falling”through oil at a specific rate. This is just scratching the surface but as you know there is more than just density going on at one time but none the less the observable universe functions without “gravity”.