Since you see transactions that occur on the card off Apple pay in the wallet also im pretty if Apple wanted they could just sell API access to 3rd parties banks and issuers already have that access possibility also payment processors.
> Apple Pay retains anonymous transaction information such as approximate purchase amount. This information can’t be tied back to the user and never includes what the user is buying.
https://www.apple.com/business/docs/iOS_Security_Guide.pdf, page 38.
They don’t have much information that the issuing bank/card network do not have already.