This is explained more in the book. The computer runs everything in the world, not just some self contained city. The sandmen travel all over the world in flying cars and bullet trains to track down runners in the most remote regions as well as cities. People are raised by robotic nannies in the nurseries, doctors are robotic, most everything is automated. There is a semi abandoned space program that runs the sanctuary program where the runners are escaping to an old space habitat colony made for hundreds of people.

The book is a lot of fun. Easy read, simple writing with lots of action based prose. The second book is a great continuation of the story. The third was not so great, but if you are reading the first two you might as well finish the trilogy. All three books are in one compilation about the size of a normal scifi book.