Ah, I think I see where you are coming from now...
I found this video that you may find interesting:
Some key quotes:
" It used to be smaller " Devs on the Hurston Dynamics building
" We only have so much time to work on these things " Devs on set-dressing of the player habs
" it's a very large interior so how much of that we can explore initially would be quite limited but it gives us the chance to show the flip-side of what people would see in the workers district " Devs on the size of the HD building and the relatively few rooms within it
" My reference library is Castle Greyskull from He-Man and the Masters Of the Universe " DiscoLando on what he referenced to draw a castle on a peice of paper, and the devs using that as an example as to why it's hard to make things 940 years in the future when they don't exist yet, like a mass transit train-like system
" We had a very specific deadline to meet with lorville so we had to make some technical and creative decisions to validate that critical path ... so we had to find that right balance between something that looked good but also that played good because we tried some designs where it was very complex and Tod was like 'dude, find your way to that gate' and it was like me being on the outskirts of London and saying find this [specific] area, an impossibility ... It was one of our first systems into making a city, we have lots of ideas we want to try in the future so watch this space " Devs on having a city wall with gates rather than other more complex open systems.
Hope the video helps answer some questions
Considering GTA5 is just one city and it took 6 years to make, I don't think we'll get fully 100% explorable cities until after the game is finished, but maybe one day... Perhaps Star Citizen 2 - gotta leave something for the sequel ;-)