Best way to listen to textbook audio files on iPhone?

If I have a bunch of mp3 files for lessons in a language textbook, what’s the best way to listen to them on my iPhone? At minimum, the app should allow:

  • Making an ordered playlist
  • Listening with the phone’s screen turned off
  • Autoplaying the next file as each one ends

This was a trivial use case before the advent of music streaming, but seems like there’s no clear default option now. Reddit threads suggest a thousand different apps, curious what people in this community like.