What is this roof constructed from?

I see what looks like gravel. If it is it is probably a hot mopped asphalt roof that has been painted with a sealer like snow coat or another sealing paint like solar flex , these are thick and many times white or silver in color. . After another look I see wrinkles around the sky light so it could be a membrane roof that someone has coated to fix a leak. Those would be my guesses a clearer close up photo would be helpful but the wide angle was not a waste.+

It looks like it could be a foam roofing material to me. That's a relatively new development though, as in less than 20 years old. It's used a lot on older buildings however because it's just sprayed on as a liquid that expands and foams up to get into all of the crevices in old tar paper and gravel or rolled roofs, without having to remove the old roofing. It also adds quite a bit of thermal insulation, typically lacking in old roofs and buildings. I've heard that now 90% of buildings in Arizona are getting foam roofing done now.

That's definitely a heated or melted seam and the only rubber looking roof that has that kind of seam is TPO (ThermoPlastic Olefin). Yours is a single-ply roof and no longer the built-up type roof that the place started with.

As an FYI for comparison, PVC (Polyvinyl Chloride) also has mildly melted seams that aren't nearly as pronounced and the material is typically white in color. There is also Hot Roll Asphalt, but you'd have a lot more seams like every 3-feet.