Volume, it is just the right mix of chemicals and metal, all shapes carry the current different, and several tests are run. One off and perfect? Not a chance.
Try Caswell, they sell kits, have some how to on line, learn the problems you are asking about.
Plating is usually copper over what ever, then polished, then Nickel, then chrome, triple plate.
You will learn about clean, pickled, etched, and the whole process is set up for the part.
By the hundreds 10% fail rate, you might get a plater. Slow as thing are, someone might talk to you if you have done your homework.
Plating gold in recesses is hard, for first everything else gets masked, then the plating has to be polished. Gold foil over glue works better. It is the way books are done.
You can get a lot of finishes on aluminum, then laquer coat for durability.
The first ones are all prototypes.