Under "normal" conditions, when I have nothing else to do my mind just "turns off" - I don't think of anything. I guess it's like being asleep whilst being awake - I take little notice of what's going on around me and take a short while to "wake up" when called etc. (I say "normal" as this is pretty much the standard state when depressed - it's very difficult to not just lapse into this "paused" state and I can't get into it when anxious).
A similar state is achieved when doing things that require a lot of concentration but not effort - such as drawing these. My concentration is focussed on drawing them to the point where I don't think of anything else (or react to the environment) - it's a kind of "escape".
Over the years I've drawn a great many of these on anything I have to hand. I usually just throw them away when done (oddly it's quite difficult to throw them away *before* they're complete). Funnily enough, I can't actually draw anything from real-life - only these things in my head. I also have no idea what they're going to end up like as I never picture them before I draw them.