What’s an amiibo card and how are they used?

So, they are cards with like codes on the back to scan to get villagers and their vans in the campground? Also, if you buy used ones online to get the exact characters you want, can you still scan them and get the villagers you want as many times as you want? Or can you only use the cards once so that used ones are basically useless? I’m confused please help xD