How to build a 3D scene

Greetings,

I wonder what's the common approach people use to quickly build a 3D scene, say, a small town with building and roads? Supposedly I have some prefabs for buildings and floors of different dimensions, how could I efficiently place them together without having to caring about overlapping? Since the only methode that I can come up with is drag and drop and manually adjust the position(knowing the size of each object), so I could make sure that the models are not intersecting with each other.

I would really appreciate any input. Many thanks in advance!