I think it's actually quite simple - unlike instagram, it's showing you videos. Videos can range from 1 to 60 seconds. Videos you like, you'll watch longer (and maybe some bonus for watching to the end). Videos you don't like, you end up skipping really quickly (and since skipping becomes second nature, perhaps you can measure the length and speed of the swipe).

Therefore, just sprinkle across various videos at first (perhaps most popular videos on the site). Show more of the videos that are watched longer, show less of videos that are watched for a shorter duration. Sprinkle in other analytics you collect (iphone user? says something about wealth and status - what videos do they like? android user? skews younger, poorer, what videos do they like? location data: show protest videos of big cities around you), and you end up with the "beautiful" soup that is Tik Tok.