Here’s how to add an YouTube video to the Watch Later list. Click video in witter → Open in in-app browser → Reopen in Safari → pull down to reveal Open in app → finally add to saved. Mere 18 seconds, ultra-fast.
Also notice how notification appears at the bottom. First time I saw it was on this video, probable because on a real phone your hand covers bottom half of the screen, reaching for Save.
And OF COURSE buttons in app are in slightly different order than in web version. Not that different for you to notice, but different enough to click on the wrong thing.