>>52509746the parts you can see in the difference are at a much lower level-of-detail (LOD) than what's around you. There are textures/models, but they're only a few low-res polygons rather than the full textures/models you get when up close.
As you approach, them, the game swaps them out for higher-LOD models/textures, in an ideal world right before you would notice the difference to prevent "pop-in", of course this doesn't always work properly (in lots of games, not just Pokemon).
Things behind the field of view of the camera are "culled", which means they aren't rendered/processed at all (or shouldn't be, anyway), or are processed minimally in memory, so that if, say, a bird is flying in the sky in the distance, when you turn around it will have moved appropriately.
Usually, things within a certain sphere/circle of the player are always rendered/in memory to make sure that they are available quickly in case they are needed (IE, the camera points at them, an enemy is attaching, etc).
The above is in general for games, it may vary game-to-game and you might see different levels of success depending on the game as well.
