Cyber Security Law of the People's Republic of China[0] give it a read. On the surface, it's like GDPR or other similar data regulation, but focus on different purpose.

The law basically gives the China government a legal foundation to block any service that does not comply with it, and most foreign service does not.

If you DO completely obey the law however, then you can operate in China. Amazon.com for example, can be accessed directly in China. But the law was very restrict and tricky, for most companies (Twitch for example) it's not worth the trouble.

(Maybe you don't know, China's game streaming service are heavily controlled: you could only stream few games that are selected by the platform. Streaming illegal game that is not approved by the authority (Battlefield for example) is not allowed and likely results a ban)

BTW: Do we have any international treaty that can prevent political censorship?

[0] https://en.wikipedia.org/wiki/China_Internet_Security_Law