Having worked at a movie theater, I can tell you that the answer is “by a human”
If you can make a machine that cleans a popcorn popper well enough to pass a health inspection, you’ll be a very rich person. Especially a small vending-machine-sized popper-and-dispenser all-in-one jobby.
I got my used laptop that now has linux on it from shopgoodwill.com I am in the US, so ymmv