[NSFW] How do these animators do it?

I am a total noob with blender. One thing I’ve noticed is that when porn animators make stuff of video game characters, e.g Overwatch, they manage to “upgrade” the textures. Like realistic skin, hair, and clothes. How do they do that, and what is that process called? Do they rip models from the game and work on them, or do they use it as a reference and build it from the ground up? If it’s the latter, is there any YouTube videos I can watch to better understand how to make existing models more realistic? Any help is greatly appreciated. I am genuinely curious.