that's basically what I experience. When I learned that many artists picture their entire drawing in detail, and then put it down on page roughly as they imagine it, I couldn't even imagine that.
i heard this from my art class teachers when I was growing up, and from a few friends who went on to be comic strip artists. However, going back and reading a few interviews of my friend, it seems like he actually imagines fairly simplified storyboards before drawing them, not a wholly formed final image.
This is not how artists work at all. I don't think any school of art has done that, other than for very rough composition (where objects go in space). Most schools of art from deep history all the way to today train with copying/sketching from life (training the eye to go where you want it), while also teaching general truths about light, color, proportion and anatomy.
People who draw out of thin air usually have these trained references in mind, so when they draw a pose they're not so much simply copying from their imagination, but from facts and past experience, about knowing about anatomy, light, and proportion. You draw as you go and see what's on the page, and you can build out a world that didn't exist, but not so much from mind's eye, but somewhere in-between the page and prior experience.
None of the above training or sketching would be needed by someone with a photographic memory or an imagination that can hold that level of mass detail (they'd only need to know practical techniques their medium depends on). There have been people like that such as Austin Osman Spare, but in the main the vast development in art has to do with learning about anatomy, proportion, light, color, and technique as taught by all the great masters.
Cartoons are a good example how a style comes not so much from the imagination, but how one very naively and simply draws characters and then builds off that natural style into something refined. This comes from playing on the page, not imagining what one wants. If you copy other people's cartoons you can also learn different lessons, and that has to do with training the eye and technique.
