I know it is not a post process thing. Is it stuff applied to the mold or stuff inside the plastic itself? I mean things like buttons, toys, phone parts before the back glass phase; things that are super thin with a finished surface that is durable and bonded to the part.
Im not a coating scientist (I’m a design engineer) so I can’t really answer your questions in detail. But powder coats are typically polyurethane or polyester. There are different types of wet paints as well. Some surfaces also get an additional UV top coat for abrasion resistance.
For a phone button you might be looking at something like PVD coatings or platings. Chromating wouldn’t be used in those cases as it is not terribly durable but it provides a good surface for the coating that goes on top or it. All of thses coatings are on the scale of a few mils thickness.
Most of these finishes are done on industrial scales and even the manufacturers send them out to external places for finishing (e.g. in Asia that type of facility is strictly controlled for environmental reasons) . At the end of the day I just speak with my supplier and tell them the finish I want and they take care of the rest.