Check out the article here (in Japanese) https://news.yahoo.co.jp/articles/07f93cc56245537b1586e0ec3c08a226dd79a5b4
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