But how can you know what is a "line/ linework" in an artwork you wish to study from?
This apparently stupid question hides an obscure truth to some extent;
In some cases it's easy to spot what is a line on heavily stylized drawing like cartoon, anime,manga,comic and the like.
And in other cases when you know it is a painting it's easy to define that such work didn't have a line.

But in cases like picrel, the bridge between line and painterly strokes is so dim that oftentimes it leads into confusion.

How can you define on overly rendered and realistic drawings, what is a line, how to determine its line quality and how can one possibly emulate such quality.