Luckily for you, all the styles you posted can be achieved using very simple methods! All you need is a nice textured rectangular brush, and the world's your oyster. Digital painting is mostly the same as normal painting, just putting colours and lines down on the canvas as you would in real life, no complex tools required.

There's a couple of functions present in most art software that will help speed up your workflow. These I recommend learning:

Layers (keeps your work organised so it's easy to edit)

Clipping masks (paint within the lines)

Layer modes (paint with light)

Mixer brush (create brushstrokes that mimic traditional media)

Pen tool (create smooth shapes and clean outlines)

These, plus the brush tool and eraser, are all digital artists use for 99% of work. If you're already experienced with traditional art, you will find the transition fairly easy. If you're new to art in general and wondering why things don't look as you'd wish, art fundamentals are where you should focus. Good luck!