How to buy products with an international card

It depends upon the products. What kind are you thinking of? Generally a card is most readily accepted in a store when you physically go to a store. You may find difficulties making an online purchase in a different country unless your account is set to that country. You would, for example, find that you cannot make a purchase in the US App Store with a card from another country unless your card was billed to you at a US address. Conversely, you could probably walk into a USA store and buy an iPhone with a card from another country.

Apple Store payment help --> https://www.apple.com/shop/help/payments

If you're trying to use the App or Media Stores, you have to be using the store for your country. If your card is from another country you may encounter problems. Apple’s statements are unclear to me. In https://support.apple.com/HT203005 Apple implies international payment types can be used when it says, "You might need to ask your financial institution to allow international purchases on your debit or credit card". Apple then counters this in https://support.apple.com/118429 with, “If you're using an accepted payment method but your Apple Account is set to a different country or region, change your country or region.”