While out and about my daughter hopped up on stage for an impromptu karaoke performance. It looks like I've filmed this on a potato but she still sounds amazing so thought I'd share (because that's what mums do).
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