Thank you!

If you're talking about the outlines, I have some custom pens for Clip Studio Paint that I've found online. For lines I use the Lineart Pen in this bundle: https://drive.google.com/file/d/1VbH_IQVDnfl-VcNV14M5CJ-kqM-nDCru/view?usp=sharing

If you're talking about the overall grainy texture on the image, that's actually a little filter I apply afterwards. I colour my characters with a basic pencil tool, and use the airbrush for soft shading. I then add a layer on topof everything and go Filter > Draw > Perlin Noise. Fuck around with the settings so it looks right, then set that layer to Overlay and bring the opacity down. It's a nice little finishing touch that gives the picture some flavour, so I always do this as my final step. :)