There’s no easy way to “mark video as watched” so that Youtube stops pushing it in your feed. (You can mark it “not interested” or “don’t recommend channel,” but those aren’t the same thing.) Even if you aren’t doing something Google would consider naughty like watching via FreeTube, NewPipe, or yt-dlp sometimes, there are legitimate cases like watching while logged-out or having a friend show you a video on their device that cause you to have seen a video without that watch getting recorded into your watch history.