>Can Facebook, Instagram, WhatsApp, Telegram, Signal, Google Chat, Slack, Discord, and others operate freely in China?

Actually yes, as long as they adhere to Chinese data laws. That's why iMessage, FaceTime, Skype, etc are all able to operate freely in China. All those laws apply to Chinese companies too.

What American data law can TikTok/WeChat adhere to to operate here? Do those laws apply to American companies as well?