The site that you mentioned uses some special kind of Algorithm named as Luhn Algorithm to generate valid credit card details. The card number is valid but you can't use it for any kind of transaction as it doesn't contain any kind of balance. These details are just for educational & testing purpose only. Recently, I found one more such website i.e. https://cardgenerator.io/ which also uses the same algorithm and process to generate card details. These kind of sites are claiming to provide any kind of card details whether it's Visa, MasterCard, Discover or any other. Again, you can't use them to purchase anything.
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