TikTok likely has enough information about others that it can begin to build a profile about you from the moment you login.

Let's use a hypothetical scenario: Someone states that they identify as a man, they're in the 20-25 year old age range, and based on phone location you can gather that they live in Texas. Now you're labeled as a 20-25yo Texas Man. Then you can look at others who fall in the "20-25yo Texas Man" category and show things you'd expect that group to like because chances are, you're more similar to others in the group than being a true outlier. If other people in the "20-25yo Texas Man" group have expressed interest in Apples, NSFW material, and lawn mowing videos, then since you're in that group, it's going to start off with that same material.

disclaimer: i've never signed up for tiktok and have no clue if this is how they do it.