Honestly, the biggest misconception is that apps can magically reveal someone’s private TikTok behavior. They can’t - unless they have full access to the person’s phone, and that crosses legal boundaries in most jurisdictions.
If you’re just trying to understand someone’s interests, check the public content they post or interact with.
If this is a safety concern (like a teen being exposed to risky content), then setting up proper parental controls or simply communicating with them is the only legitimate route.