I've lost my voice partially from being sick, sorry if my delivery isn't great and you can tell my throat is irritated. Also my bad for using mold and cast interchangeably, it's a bad habit. I'll figure out how to best use these for audio.
To listen while using other apps, switch the player to Picture-in-Picture (PiP) during playback — it keeps playing in a small floating window (the screen stays on).
To listen with the screen fully off, in-browser playback stops by YouTube's design. Open the video in the YouTube app to keep listening where background playback is supported (e.g. with YouTube Premium).
Open in YouTube app