This guy here https://www.instagram.com/kimjunggius/
As of my opinion I don't think his brush work is even possible to do. I mean he figures his anatomy, composition dynamics, number of strokes, line weight, black spotting pretty much in one go, in that one stroke. That stuff is just impossible.
I've seen most pro inkers do their stuff. This is a person for the most part of their career only have to deal with line weight, strokes, and black spotting (sometimes). even that involves an insane amount of things to get right. Most inkers attack one aspect of comic inking at a time. For example, they do overall outlines then adds in the details, then black spotting or shading. And that is just one small part of the overall drawing process.
As for his pen drawing at least I think that it's achievable. You have to deal with fewer problems like composition, anatomy, etc and I see many other artist that does freestyle drawing as good or almost good as Kim does. Guy like Katsuya Terada, most of the American Comic OGs like Joe Kubert and Neal Adams can draw great drawings in freestyle. As does many other not as well known artists, illustrators and concept designers mainly.
What practice do you guys think that'll help us to get to that freestyle drawing ability?
Or it's just Koreans were born with advanced spatial awareness because they seems to thrive in most things that require that exact skill.