I would imagine it prioritizes content that you like (as in, have interacted with by liking, commenting, etc..), and then shows the most popular videos from those categories.

I suspect it’s probably something like what @SergeantQueen describes. Also remember that TikTok is owned by the Chinese government and so you’re giving them access to pretty much your entire search/browsing history, personal images, your social network of friends, etc. If you’re running a mobile app, then your location data as well—super sketchy.