I am using YouTube website, on Firefox, GrapheneOS. If I click on youtube logo, it will close the video. what am I missing from not using the app? It also continues playing when phone closed/black screen

On the Android app it takes two touches.

Do you mean the iOS YT app? It's at most three, two if you allow a swipe. Swipe down on the video (moves it to a smaller view in the lower-right) > tap the X in the upper right of the video. Or tap the video (to bring up controls) > tap the down-arrow in the upper left (moves to the same smaller view) > tap the same X in the corner.

Full screen:

1. touch screen to see controls

2. touch lower-right -><-

3. touch screen to see \/ in upper left

4. touch \/

5. touch X

If the video play to the end (1) and (2) are replaced with touch X, for a total of 4.

I just tried on an iPad, so it's possible it's a different experience, but it was only 3: swipe down (exit's fullscreen) > swipe down (same flow as before) > X in the corner.

It is a very different experience on the iPad. I don't use YT much there, but it's not as annoying, for sure.