Sorry If the quality isn't too great I'm experimenting with the video settings, It'll take time to find the best. Hope you find a wee tingle in there somewhere :) Ohh and don't take my historical points as factum lol, Benjamin Franklyn was never President! Just a jolly clever fellow.
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