The bumper impact bar (only the top surface is visible when the bumper is fully assembled with all trim) is anodized aluminum. There are multiple components involved in both the front and rear bumpers. There is a plastic bumper skin, painted vehicle color, attached to the impact bar (aluminum part) with a bunch of plastic push rivets. Then there is a rub strip (may be either black or body color depending on how car was optioned or subsequently repainted), attached with self-tapping nuts over plastic pegs molded into the rub strip from inside the impact bar.
On the rear bumper, there are two bump guards near the center under the license plate, these are mounted with machine nuts on integral threaded studs. There is also a lower valance painted black that has a notch for the exhaust. This is attached with several screws and some brackets at the outside ends.
On the front bumper, the setup is essentially the same as the rear without the bump guards. Your car may be equipped with a front plate holder, if so it is screwed directly into the rub strip at the top, and into two metal angle brackets at the bottom.
This setup looks nice on the car, but is a real dog to take apart for repainting, or polishing the aluminum. Disassembly of either bumper is time consuming, as the bumpers need to be removed from the car to remove the rub strips and plastic skins from the aluminum portion. Getting the plastic rivets out is the worst part, they are difficult to remove without damaging them, and I've not found a really good technique for this. I use various small tools (jeweler screwdrivers, small pin punches, mini long nose pliers primarily) but they still aren't easy to get out. I'm of the opinion that GM probably intended these fasteners to be disposable, with new ones being used after a repair or repaint of the bumper skin. It just makes a lot of extra work to try and save them for reuse.
Getting the plastic push rivets out of the bumper skin is also a real job as there are so many of them. Patience is the key factor here, along with good tools and a padded surface to set the bumper assembly on while working on it to prevent scratches or other damage.
Another note is that the rub strips tend to dimple where the mounting posts are molded into the back side. Once the self-tapping nuts have been installed, they pull inward on the strip, and given the soft nature of the plastic, it deforms easily at the mount points, especially those on the corners where the bumper curves around. One needs to be careful not to over tighten and make this dimpling effect worse. As posted previously, once removed a heat gun will smooth it out again, but there is a certain technique to it and it's not so hard to scorch the plastic if you aren't careful not to over do it.
Once disassembled, prep, priming and painting is done by standard procedure for flexible plastic body panels. A good urethane paint with flex additive is heavily recommended to prevent spider webbing cracks from forming in the paint if these piece are flexed at all. A heat gun can be used to smooth out small dimples that may have formed in the plastic pieces - particularly the rubs strips - as the plastic has a memory and will return to original shape when reheated. As with all auto paint, good prep and good product is everything. Poor prep or cheap paint will render a sub-standard finish. One stage paint is not suggested.