As has been said, it won’t be reasonable to manually model and populate interiors on such a large scale. So you’d essentially need some kind of procedural approach. That way you take away the need for ridiculous amounts of level and content design, and also reduce runtime resource problems, as you generate parts of the city on demand as the player(s) moves around.
It would of course be a pretty big undertaking to write a system like that. I’m currently working on a potential marketplace plugin for building generation, but it would only form a small piece of a solution to what you want.