I would assume it's time spent on video and they start to build a profile of which users like what kind of content. X liked video 934934 so Y probably also likes that kind of video. Group people in buckets.
I'm sure this is part of it, but I suspect it goes deeper than that. I'd guess they probably have some kind automated categorisation algorithm that can extract features from the videos
The secret is how it chooses videos to recommend, not the tech stack it uses and how fast it is
I would assume it's time spent on video and they start to build a profile of which users like what kind of content. X liked video 934934 so Y probably also likes that kind of video. Group people in buckets.
I'm sure this is part of it, but I suspect it goes deeper than that. I'd guess they probably have some kind automated categorisation algorithm that can extract features from the videos