There are actually two part of mechanisms in China to wall its garden.

The first part is GFW, with which people outside of China is more familiar. It operates at every international internet cable, analyzing and dynamically blocks traffic in realtime. China only have few sites that connects to international internet, with very limited bandwidth (few Tbps in total), so it's more feasible. But overall speaking, this is the easy part.

The second part of walling a garden is about controlling what's inside the garden. Every website running in China mainland needs an ICP license from the government, which can take weeks. ISPs must be state-owned (there are 4 of them in total, no local small ISPs whatsoever). Residential IPs cannot be used for serving websites because the inbound traffic of well-known ports are blocked, which is required by the law. VPN apps are illegal. etc. These are things that are much harder to do in other countries.