Mostly of things that don't yet exist. Like a building still on the drawing board, a new ship design, or a concept design for a future product.
Most of my work is done with the help of 3D computer software. That allows me to make photorealistic images of existing as well as non-existent things, probable scenarios or even impossible ones. A ship can be made to spin around in the air or a house can be assembled from pieces in less than a second.
The possibilities are as broad as your imagination.
The workflow varies from one project to another. However, it is normally something like this:
The input for the 3D model can vary from 2D CAD drawings when available to a rough sketch or in some cases just a description or an idea.