Think of it like this, if there are beings that exist in 2d, they would never be ale to comprehend 3d, but we as 3d beings CAN comprehend 2d beings, but if we were to try to comprehend 4d beings we can't, but they can comprehend us. And as for overground and underground, its just layers of which you can percieve the world and like i tried to explain, higher frequency beings , i.e reapers and those in the game, can see lower frequency "humans/ real world" , but they cannot percieve...