It's going to depend a LOT on what type of graphical output you are looking for. Are you hoping to have realistic fur on a 3d model with simulated hair? Or is it a portrait only for a 2d game? If you're searching for this most basic level of information I would not expect to do anything that complicated. That being said what I recommend to look into this kind of thing is to research "procedural textures". You can find tutorials on how to make materials in substance designer (not painter), or material maker. A good starting point would be to become familiar with the Voronoi texture (edit: also the Musgrave texture), and displacing it using noise textures. This should be a good starting point for you. The basic idea (and this part here is jumping ahead and will make sense once you have learned about parameterized materials and those textures I mentioned) is to set up a material with inputs that you can mathematically blend.
You will also want to become familiar with vector math to get good results since colour information in these programs can be handled the same way as 3d vectors (instead of x,y,z, there's r,g,b but that's still just an array of 3 values that can mathematically be handled the same way)
The information you will come across will be for 3d modelling, but if your intent is on a 2d image you can still apply the theory.
Material Maker is free so go ahead and try it out. https://www.materialmaker.org/