The corridor, not the city, is the unit of migration

Migration looks chaotic until you watch where people actually go. They do not scatter. They follow corridors. Established routes from a cluster of home districts to a cluster of work, repeated by cousins and neighbours and the people from the next village over.

The corridor is durable. A factory can close; the corridor stays, because it is built from relationships, not from one employer.

Why it changes what you build

Build for a city and you are exposed to that city. Build for the corridor and you sit underneath every employer along it. When one factory's contract ends, the worker moves to the next gate without leaving the system.

The corridor is what makes continuity possible. It is the reason a worker's savings, his record and the life he is building can carry across jobs instead of resetting each time he moves.