If the user has enabled model inspection then there is a button in the bottom right of the viewer name "model “inspector” which allows you to view all the various channels isolated.
Anyway for the link you provided the model is not using any other maps except the base color. The reflection is just a uniform value , it looks like it is 5 or less …just about black which would be sharp reflections , mirror like. This is what is creating the effect you see. Also sketchfab engine uses IBL ( image based lighting ) so the HDRI used affects the color of the diffuse and also is what is reflected. The model inspector does not reveal what HDRI was used but im sure it is one of the sketchfab standard ones they provided which are open source on the web. So to get a similar look you need to have the model in a engine that supports IBL and use a HDRI similar to what the user used and then set your material roughness very low so it has sharp reflections.