I’ll get straight to the point
Let’s say you’re buying something for $20 Canadian Dollars (CAD) in Canada. You tap your card, and a screen comes up with big American and Canadian flags asking “would you like to pay in USD or CAD?”
ALWAYS HIT THE FOREIGN CURRENCY OPTION
If you choose “pay with USD”, the machine will take the $20 CAD purchase, and convert it to USD at a very bad rate, and then send that overpriced USD amount to your credit card provider.
If you choose CAD, the purchase will be sent directly in CAD to your credit card provider, who will then give you a much better exchange rate back to USD on your card bill.
This applies to ATMs too