China has a national security law which requires all Chinese companies to work with the Chinese version of the National Security Agency to spy on adversaries at the request of the Chinese government. TicTok is owned by the Chinese company ByteDance.
The servers for US users are supposedly exclusively located in Virginia and operated by Oracle, but the Chinese government effectively owns the algorithm and can direct ByteDance to send user data straight to the Chinese government.
There is significant evidence pointing to Chinese manipulation of the algorithm to promote unrest in the United States, promote certain political points of view, and to spy on users. One of the longer-term fears is Chinese being able to collect the data of all the teenage users in particular and use it to manipulate, blackmail, or doxx them 10 or 15 years from now as they enter the workforce, particularly government jobs.