This is an incredibly dense question the answer to which many people build entire careers out of. The most basic answer, from my experience, is that you can't separate the game world design from the gameplay, character, and prop/weapon design. For example, if you go and study an open world game environment, like the GTA…
You sound new, so let me add something. You are trying to model an entire city. San Fransisco to be exact. What is this for? A portfolio piece? If so, then pick something smaller in scope. Story time. I met a guy a few months ago, a new animator just learning the ropes, who spent a YEAR making this super shitty replica of…