I draw very fast so I have a bit of an idea It's partially just how their mind works and partially a lot of practice. They've likely been drawing or painting those things for years and years now. So it's second nature for them. Just a matter of putting together a piece that incorporates all the things they're familiar with making. Some artists want each line to be perfect. Each stroke and color without mistake so it takes them a great deal of time to paint it. Other artists don't care about the details. Using big strokes to flesh everything out and a bit of small details. Your mind puts together what it wants to see from vague ideas. Which is why sketches tend to look better than very defined and perfected drawings.