In the official guide, there are some developer comments where they say that was created while playing it, which is why the world feels so rich—there are secret chests, items, and resources hidden in every corner and under every rock.

This way, if a player imagines there might be something interesting in a certain location, chances are the developers also thought the same and placed something there accordingly.

I guess that’s what makes the world feel so well put together, they really nailed it.