Is the Apple Pay backed by a bank?

Nope! Apple Pay is just a platform to securely process the transactions from a payment method that you provide, without sending the card numbers to the retailer. It uses a unique one time code to prevent your card details from being hijacked and much safer than entering card numbers into a website.

If using a credit/debit card, they are still the ones that accept or decline the purchase and if you need to dispute a charge, it is still done the same way by contacting the number of the bank on the back of the card.

If you’re thinking of Apple Cash, rather than Apple Pay, it’s issued and serviced by Green Dot Bank.

https://applecash.greendot.com/termsconditions/

Apple Cash - Apple