Creating an explorable life like city is extremely time consuming. I have no doubts that the 'Vehicle Switching' template took dozens of hours to complete. I've taken a few shots at creating a city myself, it's not easy, you need to think about and create the city's..;
* Layout; roads wide and narrow, which areas; residential, commercial, industrial etc and then put down the specific building shapes!
* Textures for the buildings, streets, graphics and animations for the vehicles and pedestrians, the list goes on!
* Traffic management; traffic lights, create a continuous flow of vehicles and pedestrians to make it feel world like
* And if you make it this far then you've only completed the city itself! You will still need to think of a theme, a storyline and code, write and create the missions, and create a system for money, weapons, enemies the list goes on...
This will take many months if not a year or more if you want something expansive and quality. I mean it's doable, but you'd best team up or have a lot of patience and persistence.