This forum is not for tutorials ! read the rules before posting something, you have a lot of tutorials on youtube, google and other websites so try to understand, because is not the first time when we see you don't want to understand.
I have searched the whole internet, there is no relevant teaching material, I can think of what you can think of, It’s because there are no tutorials on the whole network, so that's why I came here to ask for expert advice.
I know how to paint realistic fur, I can draw it with Ps, but I want to know how the fur is grouped and how to apply it to the perspective, and also how to fit the skin in illustrator, there must be some tips.
I know how to paint realistic fur, I can draw it with Ps, but I want to know how the fur is grouped and how to apply it to the perspective, and also how to fit the skin in illustrator, there must be some tips.
There aren't.
If you can still get it, try the book "Illustrator WOW".
There is at least one example image of a cat in it (in the CS version of that book if I remember correctly). Don't know if that one comes near what you expect, because it doesn't actually detail the fur. If you know how to draw fur then all I can is advise you to apply the principles to vector drawing. There is no short track.
Do the rendering of light and shadow using whatever technique pleases you. Apply detail using brush strokes to whatever level is appropriate. That's it. You have to find the way yourself.
As I said: it contradicts every vector principle. It contradicts the reasons why people use vector graphic at all. Doing a tutorial might bring you some admiration by experts (or maybe not even that), but there simply is not enough demand for it, because nobody wants to actually do that.
I wouldn't exactly call that realistic. Please just do a search for tutorials in your favourite search engine. It's not something a lot of people will even try, so I wouldn't expect too many results at all.
As for your other question: "rendering the skin"
Which skin? People? naked cats? zebras?
And what exactly is your issue with it? Drawing at all? being realistic? the lights and shadows? the structure?
Please put in some effort, because you will need to put in even more effort when actually drawing it.