I had this exact thought going through Solution 9 and couldn’t figure it out myself lol.

Looking at the ‘apartment buildings’ in the distance would make you think there’s like, millions of people in Solution 9, definitely more than any city state as you say.

But then from what we know most of the people in solution 9 are either the remnants of what was once Alexandria or people from Tuliyollal who got sucked in by the bubble. Which doesn’t account for anything close to the amount of space we’re shown lol.

Only headcanon theory I can think of is that they planned at some point to start projecting the Endless into Solution 9, which would essentially give them an infinitely growing population. So maybe all the ‘dead space’ is for all the people who were meant to come back and live again