It depends, if you want to build a city you can either do so procedurally (which also allows endless regeneration) or you can do so manually. But even if you do so manually you will likely want to duplicate a lot of things, for example you probably don't want to model each lamppost individually but instead keep one model for all of them. You can do the same for entire building especially when you copy over a building and then make some small changes (change the number position of tables in the houses ext).
But no matter how you do this you will have to model every object.
A good system if you want to do this procedurally is by first generating a region map (this part is rich this part is poor industry goes here and shops there). Then one you know what region you are in creating a road map. You can then divide the free regions in areas where buildings go and then look into a folder with different buildings you have created, each building in this folder should contain regions and parameters for where various objects inside the building can go (and maybe even make the buildings slightly dependent on the space they occupy so that a 2 by 3 version of the building is slightly different but still generated from the same basis as the 3 by 3 building). You then add is details to the streets and maybe some people and you are done.
Be warned that if not carefully managed a procedural city can end up feeling very bland. To prevent this apply the following techniques:
1 many different types of regions, to somewhat offset the cost of this remember that you reuse some of your old buildings.
2 a great deal of variation between regions (and maybe between streets). This can be the simple things like changing street lights or by making different cars stand in different regions (what is a Ferrari doing in the poor part of town?)
3 some uniqueness to regions, allow buildings/objects that normally shouldn't be that this can create a feel that something is going on here, maybe that is and old factory standing near the rich part of town, maybe that Ferrari standing in the poor part of town really does belong there (just a lot fewer then in the rich part).
Manually you will likely want to follow the space steps but instead of having a computer do it you do it.