How to properly use coupons?

Aren’t cashiers supposed to keep track of which coupon goes with what item? How do they know there’s not two coupons for the same item? Usually you’re limited to one coupon per item.

I’ve been watching this show on tv Extreme Couponing for a while now.

From what I’ve seen so far, the cashiers aren’t keeping track of the coupons that belong to the item it was intended for. And these people on this show literally have thousands of coupons to use.

How do they know they’re not scanning competitor’s coupons? Are you allowed to use one store coupon in a different store?

Maybe I’m completely wrong about all of this, but I detect something fishy is going on.