@freebliss Everyone else has basically covered what I think, but I'll add that for me "Theatre Mode" on YouTube desktop is mostly about pushing the right-hand sidebar filled with algorithmic suggestions under the video. Sometimes with livestreams that's a chat replay instead.

I wish Theatre Mode was a user default. Recently YT even hide the UI toggle (A/B testing? 🤷‍♂️), only allowing a key command to change viewing mode; They really want to promote that right-hand sidebar content!