There is some confusion about the default card you are talking about. For the charges made with apps that are managed by Apple, that is with the payment methods on your Apple Account and you can view or change that with the link below. More specifically in your Apple Account you can have a list of payment methods to be used and the first one on the list will be tried first and if that is not able to complete the transaction, then it will move down to the next payment method. The money in your Apple Account will always be charged first, so that one will always be on top to clear any Account balance if you have one:
If you need to change or update your Apple Account payment method - Apple Support
Apple Pay also has a Default Card to use in your Wallet when you choose to make a purchase using Apple Pay, That can be changed here:
Change or remove the payment cards that you use with Apple Pay - Apple Support