Jumping in to say: even the monitoring tools that claim to offer “stealth access” are extremely fragile.
iOS updates break them constantly, and Android security patches flag them. Not to mention you need physical access to install them.
Anyone promising invisible, remote TikTok monitoring is overselling. 
If you want to keep an eye on trends or content types a person might be seeing, try exploring TikTok’s “For You” categories or using a logged-out browser.
It gives you a rough idea of what the general algorithm for similar demographics surfaces.