Many artists aren't trying to simplify an final subject. If anything, they come from the opposite direction - they don't worry at all about the final object - just look at it. Look at its lines. Mark out the major shapes. And there you have your abstraction. Now if you want to finish it, keep adding the next layer of detail. Mark out the smaller shapes. Mark out lines and endpoints. Start filling in some shading. Then add some texture.

In other words, you start from a simple abstract and fill in. And most of the 'how to draw' sketches from the original post do exactly that.

It's interesting to compare how theatre went from scripted shows to improv. Soon painters will ask audience members to draw some random shape for them to start.

I love doing this when doodling. I can start with nothing, but often it is more interesting to start with a scribble!

I have observed the same (abstracting the essentials) in some of young children's drawings. They seem to be adept in identifying the essential elements from the details.