To those who are wondering what apps I use.
I mainly use Autodesk's Sketchbook as my core app (free version) for my line work and painting. So I only have 2 main tools I use, which are the pen tool and an airbrush. Also I'm forced to work only in 3 layers, since it's a free version. I divide it by 3 parts: Outline, base paint and the edit layer.
The outline layer is self explanatory, it's the layer that's always on top, so I don't paint over it. I also use my eraser to taper my lines.
The base layer is where the overall color of the work is settled down.
The edit layer is where I create depth, shadows and highlights, which is painted in that order. When I'm happy with the result of my edit layer, I merge it down my base layer, and carry on with the next parts.
For my backgrounds I either merge everything down and paint behind it. Or I just use my Trigraphy app. For HQ edits, textures and design.
Those are the techniques I use for all my art.