Eli5: how does platforms like Steam or Amazon know if you’re using a correct visa card or not if you didn’t buy anything with it?

Question

When you enter your card details to add it to your account and you write 1 number wrong, it instantly tells you that the card is wrong. How do they know?

In: Technology

Answer 1

I believe similarly to an American SSN, the first 4 numbers of the card are the bank that card is processed through. For example 4833 is Visa where I live. The second set is another specific set of numbers and I believe only the last 4 really specify YOUR card. Hence why when you go to online checkout you see your MasterCard and it only shows the last 4 numbers because that’s the only info you need to distinguish one out of many cards.