I can refer you to Bill Perkins who explains styles and the language of Art in the best possible way. if you search his name you'll find his online courses.

Hi @Mary Santos. I don’t think you should worry about one type of technique being inherently superior to another. Working with line only is neither more nor less professional than using other techniques, even in the gaming industry. What matters is the quality of your work (cheesy, but true). Also, remember that it is easy to forget that professional illustrators work differently than those starting out. Just because we don’t see lines in the end result doesn’t mean these artists 1) don’t use lines in their process or 2) that they don’t apply the understanding of form and perspective that comes with knowing how to make good lines. Whether your line art is meant to be the entire illustration, a part of it, or not show at all is mostly a matter of style. You’ll find plenty of professional artists that do any or all of these. There are some wonderful digital illustrations used for game art, but that doesn’t leave out line art entirely – just take a look at Hollow Knight, the Tell Tale games, Borderlands, etc. I’m not sure if this is part of your question, but here it goes: Your drawing is of a similar style to what a friend of mine used to do. In her case, whenever she made her drawings neater by erasing the sketch underneath, it became flatter and much less interesting, so she left the sketch in but was never happy because her drawings were never as neat as she wanted. It turned out that her final lines –the ones that were not meant as sketches– did not show what they were meant to show: shape, light, a change of material, movement, etc. The sketch lines carried most of the weight, so erasing them erased the drawing’s most important information. Shape, light, etc. are things shown in the illustration examples you shared, but they are solved in a painterly-language, not in a line-language. Line art can show these things as well, but you have to solve them differently with line than you would with brushes and color. If you are interested in improving your line art, I recommend taking a look at what fine artists or comic artists do. Their lines aren’t always meant to do the same things, but the crucial idea is that they know what they are doing with their lines and why. It’s great to have digital paintings as your end goal, but don’t underestimate line work!

Okay, just because your favorite artist does it a certain way, doesn’t mean that’s why their art is good. There are plenty of amazing artist in the game industry that use line art. Do yourself a favor and look at an artist named John Grello, very few people can draw or paint like that guy. if you do you really great, then your stuff will look professional, line or not.