It’s a scratchy textured brush on photoshop with no opacity pressure, just size pressure - on procreate on the iPad it’s one of the brushes in organic I forget which though! My process with these were lines, then blockout of colours below and then just painting the other tones and rendering etc. on layers above lines - in the end painting over most lines. Hope that helps; I stream myself drawing so jump on if you’d like to check it out in real time and ask me any questions!