the model itself is taken from smutbase and in addition to the problem with transferring bones under the unit, we are faced with problems with textures, or more precisely, skin!

Its textures are divided into four separate textures, but the first three appear to be used. Judging by the themes, I need to combine these textures into one, meaning bake them. But I simply can't bake it. Blender says the texture is sealed, but it's just transparent.
I thought about the textures that it would be worthwhile to simply combine them into one material in Unity itself, which I did, but it displays completely differently.

I decided to combine them together, but I see some unpleasant un-glued parts of the model (by the way, the black spots are not the transparent part, I cut it out in Photoshop)
I looked at various guides, including the most obscure ones, but there is no answer in my specific situation.

By the way, the model already uses a metarig, but it is not complete; it only has the body base, but no bones, hair, jacket, etc.

Only Rig himself has them, not Metarig

Please help me in any way possible in my case.