Every title starts on Auto, which uses the built-in player. If the mpv add-on is installed, Auto switches to mpv for videos, audio or subtitles the built-in player can't handle, and goes back if mpv can't play them either.
- While an episode plays, tap the settings button at the top.
- Under Player, pick Auto, Built-in or mpv.
The choice applies to the whole title. On Auto, the same row says which player is in use and why.