You can view SkinGen as a tool to add details to the skin as well as apply makeup. Once you are finished and, exit the Appearance editor and the textures will be "baked" and converted into Digital Human Shader materials.
If you want to change something and open the editor, the reverse process takes place and all the SkinGen layers are again available to be modified.
The skin of any existing CC3+ character can also be enhanced by opening the Appearance editor, and adding layers for wrinkles, makeup, tattoos, etc.
CC3 type characters will need to be converted to CC3+ first. For those characters, a button allowing to you convert will appear in the Modify panel.
It can indeed be overwhelming when you are just starting out and even if you are an old hand like myself...
So asking questions is good as it helps us all learn.