There are a bunch of simple and not so simple models shared on the web you can start with if just want simple game assets.
For my classic architectural designs I model them. There are a few techniques on making it faster. F2 is my friend for my modeling workflow.
If you don’t want to model, there are many other methods of bringing in geometry into blender:
Importing svg’s is a great method though can get complicated with weird geometry as blender interprets the data (a lot of verts for complex images).
Textures using displacement as was mentioned earlier.
Node manipulation for generating textures automatically.
Animation that builds geometry.
…Just to name a few.
The way I do it is to render an image and make normal maps from the models. I am sure there are half a dozen other ways to do but since I have a ShaderMap Pro I need an excuse to own it. There is free software that does the same.
The left image is a 3d model and the image on the right is a plane with the normal maps added.
This model was created using an off set hook command on a single shaped line and then extruded around a center.
That just gave me an idea. Back to the experimentation board.