There are more than enough sources, just google "TikTok Chinese government influence". Just a few examples:
https://www.forbes.com/sites/iainmartin/2023/07/26/tiktok-chinese-propaganda-ads-europe/
https://www.axios.com/2024/03/11/tiktok-china-us-elections-influence
Also, just think about it: The CCP loves spreading propaganda. There's a massive social media platform controlled by China, which is used by young people in foreign adversary nations. Why wouldn't they leverage this platform to spread their lies and influence people? It's literally the perfect opportunity.
Uh, no? I don't know who told you this lie, but https://github.com/signalapp/Signal-Server
Btw it wouldn't even matter, since the encryption happens in the client app. The server basically just passes around encrypted pieces of data between devices.