I mean, yes, the great firewall is sophisticated. That example though is probably the most basic feature of the most trivial firewall possible, your home modem could probably do it.

More interesting examples would be some of the clever packet analysis they do on encrypted connections.

But again, even this isn't enough to prevent all undesired traffic, especially given that there's legitimate uses for VPNs, overseas file sharing sites, etc.

For instance, if they blocked external access to every server in China they'd be knee capping their entire economy, so they have to work out which ones. That's not easy or cheap to do accurately at scale.