https://en.wikipedia.org/wiki/Great_Firewall:

“The Great Firewall operates by checking transmission control protocol (TCP) packets for keywords or sensitive words. If the keywords or sensitive words appear in the TCP packets, access will be closed. If one link is closed, more links from the same machine will be blocked by the Great Firewall. The effect includes: limiting access to foreign information sources, blocking popular foreign websites and mobile apps, and requiring foreign companies to adapt to domestic regulations. Due to the Great Firewall, China has one of the lowest cross-border internet traffic rates in the world. Usage of foreign apps in China is minuscule; Asia Society estimated in 2026 that foreign apps blocked by the Great Firewall have extremely low traffic, particularly compared to domestic apps; the top five domestic apps saw traffic that was 1,000 times more than the top five foreign apps.”