Lol. This is months old. Just refound it on my phone and thought why the heck not. Made during an extremely slow day at work. I was large animal filing and then Jackie decided to come say hello. Mewmewmew.
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