Watch history

YouTube keeps a per-account log of the videos someone has watched, and the recommendation system reads it alongside search history, subscriptions, likes and satisfaction surveys when deciding what to put on Home and Up Next. It is personal, so two people looking at the same watch page can be offered completely different things afterwards. Creators cannot see any individual's history, and nothing in Studio exposes it.

The practical consequence is that your video competes for a slot against whatever else that specific person already watches. That is why the same video can spread fast in one pocket of an audience and go nowhere in another.