Not sure what you’re asking.

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.

[Fullscreen icon in YouTube is double arrow, not broken rectangle.]