I have a library of hand painted textures that I've created over the years (creating new ones when needed). I sketch, the paint (ink) over the sketch, add textures, then color. Voila!
This is a sketch for a background from last season's Squidbillies. I
needed to create a super mall for a simple cutaway shot. It includes
sports arena, landing strip, it's own water supply and interstate that
enters the mall itself.