Apple Pay is simply the ability to use your stored cards to make payments via your physical device, app, or website. Your Visa card is issued by your bank.
The Apple Pay card details (shown below) are simply to provide four digits in case you need to verify a transaction. This is unique to the device (not the card) and does not change.
When Apple Pay is used it doesn't share the physical card number (one of the reasons it's more secure), as such, if you're asked to verify a transaction using the last four digits, you may need to provide this number instead. Again, it is not an actual card number issued by your bank.
Hope that helps!
Note: You may see a different four digits for each card, this associates it with the card but is still unique to the device used to place the transaction.