The crinkly gold stuff is aluminized Mylar. That brand of Mylar is naturally yellow, and it's installed with the Mylar side out to provide the proper optical properties for managing radiative heat transfer. It's installed by hand, and hand-crinkled to minimize conduction paths between layers. You are seeing the outermost layer of a "blanket" of Mylar and/or H-film about 18-24 layers deep. The many layers provide the same mode of protection as laminated tank armor -- layers of barriers strong enough to fracture incoming material, separated by empty space or softer material to allow the fragments to diverge before striking the next layer. This has the effect of spreading out micrometeoroid impacts.
The aluminized Mylar is mostly on the descent stage. In some cases the outer layer of the blanket is black H-film. The H-film is used where additional thermal insulation is required because of engine plume impingement, or where less aggressive optical properties are required (e.g., in the right rear quadrant). All that stuff is taped in place using tape made from the Mylar and/or H-film itself. It's as close to duck-tape as you can get without it really being duck-tape.
The ascent stage skin is sheet aluminum alloy, cut roughly into panels and fastened to the underlying framework using fasteners akin to rivets. These panels are neither load-bearing (except as they contribute to the monococque design) nor airtight. They are simply the thermal and micrometeoroid shield for the upper stage. The fasteners incorporate thermal insulation to limit heat conduction to the structure. In some areas the thermal design is changed to increase the heat transfer to the interior. The hypergolic propellants must be kept essentially at room temperature, and so a pattern of light and dark panels was devised to provide proper heat transfer characteristics in a variety of view factors.
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