It's not exactly a question but I'd like to point out that metalness is not the reflectivity.
As for how Unity's standard shader works it uses the metalness value to overlay the albedo with black and then uses the albedo color as reflective color. Usually this is a 1 bit mask because materials are either a metal or not.
Your metalness map is in fact a roughness map which describes how rough a surface is where white is non-reflective and black is fully reflective. You would have to invert your metalness texture and put it in the roughness slot.
I think you can combine metal and roughness where one texture occupies the rgb and the other the alpha to save on texture memory.
