Algorithm, Search & Distribution
Recommendation system
Also called: recommender, recommendation engine, recsys
Everything on YouTube that picks what a person sees next runs through it. It works by automating word of mouth. When the system sees that a viewer enjoyed a video, it offers that video to other viewers with similar interests, and when someone rejects a recommendation it becomes less likely to show them anything like it again. Alongside raw watch time it weighs measured satisfaction, gathered from surveys asking whether people felt the time was well spent.
The common mistake is treating it as a judge marking your channel. It responds to how individual viewers behave, so a video earns its distribution from the people who have already clicked on it.