I didn't really have the time to read through your question entirely, but I did skim it and what I determined you to be asking is basically this: how much, and how often, do artists rely on the kinds of techniques we encounter in this course, and in others, to lay down construction and guidelines for their own drawings.

The answer is as much or as little as they want. You've got artists like Kim Jung Gi, who've such a strong internalized sense of space and such a broadly developed visual library that they can whip off things with no construction, no underdrawing, just one and done. Then you've got artists who will apply construction far more frequently and thoroughly to help inform the choices they're making. Although even talking about it as "this artist does this, that artist does that" doesn't really make sense, because it's situational as much as anything else. For example, there are some things I'll use more construction for, when they're particularly tricky, and other things I'll just whip off more fluidly. Most of the time I sit somewhere in the middle.

All of these techniques and approaches are tools in your belt. As far as Drawabox is concerned, it's using construction as an exercise towards developing a very specific skill, but that doesn't mean you can't use that exercise as one of your tools. Furthermore, if you want to put your all into a final piece, of course you'd use every tool at your disposal to achieve the particular outcome you desire - from doing preliminary studies, thumbnailing, gathering ample reference imagery, and other kinds of preliminary planning, to using construction, underdrawings, and other "stages" in the piece itself. Hell, you might even incorporate elements of photo, or 3D models.

I assume you're asking the question because you've primarily been exposed to art as performance - live demonstrations, where the goal is less about the end result, and more about the video recording, and as such, your understanding of how one might go about it is a little skewed.