Often artists create the environment concept in the following way foreground - 3d rendered, background - photomontage / mattepainting. Sometimes elements impossible to achieve wit photomontage in the backgrounds can even be added with 3d software.
Here is a selection of matte paintings by Wayne Haag, he implements a lot of 3d with photomontage. Often foreground buildings, vehicles and objects are created totally with 3d software and the rest with matte-painting techniques.