On a) if the vast majority of users end up watching the same videos then they wouldn't need a fancy rank system and instead rank videos by most viewed per day,

On a) if the vast majority of users end up watching the same videos then they wouldn't need a fancy rank system and instead rank videos by most viewed per day, or something like that. My gut feeling is there's ways to segment users by their views and infer many characteristics of the videos a user wants to watch by looking at what similar users watch. It's still highly dimensional though. b) My guess is they're optimizing for viewership engagement with the added side-benefit of video-creator engagement.