Source: https://github.com/jb1420/YTMAmbientDisplay (MIT, no build step)

To test: install, open https://music.youtube.com, play any track, then
press the fullscreen button in the player bar (left of the expand button),
or press G. C toggles the lyrics panel, Q the queue.

The only network request the extension makes is to lrclib.net, to look up
timed lyrics by track title/artist/album/length. It is declared as
websiteContent and can be turned off in the toolbar popup ("Synced lyrics").
No background script, no remote code, one permission (storage).