it's built on the model, when is made we use an "interior" model wich contains all de virtual cockpit (or not) , and the visible (from the cockpit)  outside parts and another "exterior" model wich is the whole aircraft that you see in the external view of the FS, if the author didn't make the interior model it won't appear of course, some authors dont make virtual cockpit but left the wings to complement the angle views of the 2D cockpit, some very skilled and talented (not like me :p) complete their virtual cockpits to give us a full experience, and a complete aircraft to enjoy .... and no it can't be modified unless you have the Gmax or FSDS original.