You can always choose your desired payment method at checkout. Usually if you have a mobile wallet set up (Google or Apple Pay) it will default to that payment method, but you can adjust by tapping directly on the payment bar, and then choose the option to switch from the drop down menu.
You can adjust the card you are paying with through Apple or Google Pay at checkout when you confirm the payment.
When you are given the chance to double tap or enter passcode to pay, you can tap on the payment details to adjust the card. Please note, some banks do not support in-app purchases.