Another way for a coin to become shiny is polishing, which can cause a similar appearance to plating if done well. That could explain the visibility of the copper core.
Modern US coins aren't plated. They're struck from metal planchets/sheets whose composition is created prior to being sent to the mint. All the mint does is strike the coins from the planchets/sheets it receives. Any electroplating (like those done to copy "coins" by private companies -- like those you'd see in print ads or infomercials/commercials) would show an even metal over the entire "coin."
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