UMii files have many of the same attributes as Mii files, and the game interprets those into the characters that you can see in game.

Image: characters in Breath of the Wild aren’t exactly Miis, as you can tell by just looking at a picture of them. Obviously, the game doesn’t render the characters exactly like Miis. It’s been speculated before that Breath of the Wild’s NPCs were somehow related to Miis (especially after the discovery that their game files were labeled as UMiis), but Alice’s discovery reveals that Miis can be directly mapped into the game, with only a few exceptions. As with all systems like this, there are edge cases and caveats