I realized the sound quality is not so awesome - please excuse me for that! I had my phone placed on the deska together with my working laptop that makes the buzzing noises ugh. Well anyways hi!
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