: You can use access codes to download custom Mii creations from other players. How to Make Mii Characters in Miitopia - Play Nintendo
If you are using this file for a modded console or emulator:
: This is a digital game format for the Nintendo Switch, typically used for digital titles or eShop downloads. In the context of modding or emulation, these files are often used with homebrew tools or emulators like Yuzu or Ryujinx. About Miitopia for Nintendo Switch