Getting official residency in a city requires applying and fulfilling whatever (rather arbitrary) requirements the local government imposes. It's in their best

Getting official residency in a city requires applying and fulfilling whatever (rather arbitrary) requirements the local government imposes. It's in their best interests to be picky about who they let in, so the bar is set very high; Usually you need (1) a local employer to sponsor you, or (2) bribe one of the right office employees. Both of those are beyond the means of most rural peasants. Those who have the money to afford city life but not the right connections to get the right papers will move in anyways, technically illegally. As long as you don't cross the wrong person and get caught, you can often survive that way for quite a long time. Disclaimer: My Mandarin is awful, and I'm almost certainly glossing over a lot of details, but that's a rough summary of what I've seen.