I used Cinema 4D to create a unique environment for my story. I wanted Block City and everything in it to be unique. And though I based a lot of the material on existing references I needed to create most of my own. So from my sketches I constructed element in the virtual 3D world, lit them as you would a stage or movie set, moved my virtual camera to where I wanted it then created jpegs. I then used these as a base reference to draw from.
Other illustrations were based more on photographic reference I had taken though often included the CGI elements.