Can anyone explain the process to a simpleton like me?
Powdercoating melts a plastic powder to form a very tough coat.
Paint (or enamel) is solvent based (might be a hydrocarbon solvent, or might be water). Some are a reaction and made up of two parts (2k) which when mixed set. Paint is almost always the better quality finish (you wouldn’t powder coat a car for example), but it is more fragile and chips.
Metalic finishes are made by adding various amounts of metal dust to the clearcoat on top, not the color otherwise you’d not see it. So strictly these aren’t powdercoated, but can be applied on top of powdercoat as long as the temperature they’re cured at is less than the powdercoat melts at.
Candy finishes are a dilute paint in clear lacquer over a silver or gold colored base coat (although you can do it with a colored base coat and top layer).
Pearl finishes are like a cross between the two, except using mica (ground up white rocks) not metal, in an otherwise transparent colored layer so you get a slightly milky color.
Hence why some powder coaters don’t offer them, because they’re not setup for doing clearcoats on top of their powder.