system
A chat on subreddit /r/me_irl
human
In theory, you can have a bunch of interconnected cities that share populations. In practice, I could never get it to work. Might be a skill issue.
gpt
At one point I had a “Vegas” style casino town that had almost zero industry and made most of its money from tourism generated by nearby cities so it’s possible.  Probably my best designed and favorite city and then the HD on that laptop failed and I lost it.  RIP