Blender Character / Tree / Item rendering orders & Unity imports?

Hello, having a few issues understanding what to do, in order to colour an item/ character. I’m having an issue with understanding the order of “painting” etc. If the character has skin, it needs a skin texture, then if it needs scars, a scar texture, then so on & on. This makes no sense to me, as how do I put the texture in the correct limited position, like a scar on a face? How do I colour a weapon with multiple colours for different parts?
I have seen some methods for cutting out body parts/copying/ to make clothes, but the same problem still exists, how do I shade a body part and not all of it with one material/texture etc?

I also have an issue with what to do in the correct order, if I have done everything,to import a tree with transparent leaves & bark textures/materials into UNITY Game Engine without getting a grey tree with white pages attached to it . Do I need to bake in cycles, or save different materials in different ways? attach armature? & what file type works?
I know this is a lot to ask, but this is probably the most appropriate area.

UV mapping is used to do that. An UV map contains UV (XY) coordinates for every corresponding polygon. UV unwrapping is done to get a useful layout on an image, which is where the pixels on the image are mapped on the mesh and vice versa.

Image to mesh when the image is used in a texture, which in turn influences various material properties. When you have an image that is made to control a specific material property, it’s called a texture map, usually named after which material property it controls: Diffuse map, specular map, mask map, bump map, normal map, albedo map, glossiness map, metalness map and many others, depending on the material system.

Mesh to image when you’re painting on the model itself, the result goes onto right places on the image with the help of the UV coordinates. Same when texture baking.

UV unwrap is done after modeling. The UV maps are also part of object data, in this case mesh, they export with the model geometry.

Yes. Cycles materials don’t export. Blender render materials do, but the material definitions might not be suitable for the target material system. In that case you have to build the materials in the target application using the texture maps you’ve prepared, or use a third application for texturing.