Did you ever take a chemistry class in school? Or a metal shop class?
When I took those classes 40+ years ago, we did learn basics of both chemical plating and electroplating. I can still turn a coin to gold with a little zinc and common household chemicals. Can make the same coin look silver, too. We had a lot of fun doing it back then. I would guess that some schools still teach these fundamentals, but I could be wrong.
As far as how many plated Kennedys are out there, I have to agree with Arkie. Probably more than the entire 1970 mintage. I find a lot of gold plated Kennedys out there too, but I do not get excited about those. It costs nearly as much in time and trouble to deplate them as the gold is worth.