You can get ad-free, fullscreen video on mobile right away by opening it from a chat message preview. You also just get the full-screen video, no way to minimise it and see Youtube UI, no ads visible. I was just wondering how it works and can you simulate that on the desktop.
You can get fullscreen video in a desktop browser by pressing F or clicking the fullscreen icon (broken rectangle) below the video.
You can get ad-free playback by paying for Premium or sometimes with an ad blocker.
Outside the browser, you can get both with yt-dlp, which also integrates into video players like mpv.