Well, the exact way is secret of course. And shouldn't be digged at too hard lest the Chinese government just stumble on a reddit post explaining it and then fixing it. But basically, no wall is perfect, there's always holes if you look hard enough. Even the Iron curtain and Berlin wall let some people through. But the great Chinese firewall do nothing about passing along physically. For example, as soon as a thumbdrive reaches a computer not governed by that firewall, uploading is simple. There are also ways to bypass firewalls, every lock is beatable, it's just a question of time