I downloaded two assets from the Unity store, with the first one I built a modular character (head, torso, legs), then I took another one from the same author, to have more body "modules", but the body parts are not perfect in the joints, due to mesh intersections (ex: from head to body, the neck doesn't fit right, see the first image).
To have interchangeable body parts, I only replace the mesh of individual body parts (for example the head), and it works with the original FBXs (of both Assets downloaded). I can replace the mesh on the Skinned Mesh Renderer component and have a different head.
So what I did was edit the Asset Store FBX's in Blender and import them back into Unity, but the result is that of the second image.
This mesh comes from one of the two downloaded assets FBX, and it works (even if the neck is not perfectly aligned).

This mesh (same name Head Female (Bald)) comes from an FBX that I saved from Blender. In this case, to remove any doubts, I took the original FBX, I imported it into Blender, without making any changes I exported it into FBX and imported it into Unity. This is the result:

Note: The FBX I import from Blender, if I put it in the scene as is, it is visible with all materials.
Where am i wrong?
Thank you.
Sorry for the first version of the question, it was confusing and I hadn't written a key part.