After creating one with a dedicated console app, they can be used as characters in Wii-labelled games such as Wii Sports, Wii Fit, Wii Play, Wii Music, Wii Party. Other games that support Miis are Mario & Sonic at the Olympic Games, Guitar Hero and the more recent Mario Kart and Smash Brothers games.
Wii consoles can store up to 100 Miis, up to 10 of which can be favorites. The Wii Remote itself can hold up to 10 Miis on its internal memory, which can later be transferred to another Wii console. However, they can only be edited on the console on which they were created. In addition, Miis could also be exchanged via the WiiConnect24 service, thereby allowing a player's Miis to "mingle" will enable such Miis to appear on other people's Wii consoles. Users can also transfer Miis to supported Nintendo DS games. The Miis can also be arranged in different orders.
Wii U consoles can store up to 3,000 Miis, up to 50 of which can be favorites. It's possible to import Miis from the console's Wii mode. It's not possible to store/read Miis from Wii remotes, not even in Wii mode.
Miis can be transferred from Wii to 3DS and between 3DS and Wii U. 3DS and WiiU can import/export Miis as QR codes. An amiibo figurine or card can each have a Mii stored as its "owner". A linked Nintendo Network ID can store online the Mii used for its console account.
The Nintendo Account allows creation and cloud storage of up to 8 Miis, with one being used as an avatar. It's possible to import the Mii from a linked NNID.
The smartphone app Miitomo could also import/export via QR codes and use its main Mii for the Nintendo Account being used, including the current outfit worn in the app, for a time.
The Switch can use the Nintendo-Account Mii and other than that, only read/write Miis via amiibo.