You might have seen that in the Content tab, you can access advanced material options.
Accessing this area, it enables to work more deeply on the texture material of your 3D object(s) such as the Albedo, Emissive, Ambient, Reflectivity, Bump and Opacity. Read more in this Helpcenter article if you want to know more about the difference between all these maps.
Where to upload some textures?
If you're looking for some 3D texture libraries, we suggest to go to: