Better document ExoPlayer concepts (e.g., TrackGroup)

Hi to all!

Thanks for working on Exo Player, It really provides lots of functionality to work with.

What I am finding very hard is that, the documentation https://exoplayer.dev/track-selection.html exists only how to bootstrap or play a video or main features it supports.

Is there any documentation like, What is a TrackGroup, SelectionOverride, Events and more stuff.

Comments or answers like, see the DemoApp are very boring already since no one wants to study the whole Demo App to find out what a TrackGroup is.