I joined for the first time a few days ago using my gmail account and then selected a few tags like animals, comedy, sports. I only watched about 10 cat videos.
Next day I get a notification of a video called "I'm in my 40's, single and I don't have any children". I was shocked.
It's impossible for their algorithm to deduce so many facts about me. Which leads me to think that I already have a shadow profile with my gmail address and/or TikTok is somehow working with Google on this.