Why do countries use propaganda? Why do companies use it? Why do people use it?

Because changing what is important to a group or people can make pushing your agenda easier.

What would be important to China's propaganda wing? Most important would be to maintain control over China and downplaying anything that reflects negatively. So controlling negative speech or shifting blame to someone else if a covid virus is discovered to original from territory you control would be important.

Tiktok's algo focuses on keeping you occupied in a state of mind where you zone out.