Hi smcrea,
In order to use Apple Pay, you would have to use Face ID on your iPhone. More info can be found here: Make purchases using Apple Pay and below.
"Pay with your iPhone
- To use your default card:
- If your iPhone has Face ID, double-click the side button. If prompted, authenticate with Face ID or enter your passcode to open Apple Wallet.
- If your iPhone has Touch ID, double-click the Home button."
For Family Sharing, the group will use the payment method you have setup on the account as the organizer. More info can be found here: How to share purchases with your family and below.
"Which payment method is charged for purchases that family members make?
When Purchase Sharing is turned on, everyone's purchases bill to the family organizer's payment method.* If there's an issue with the payment method, the family organizer can update their payment method or add a new one.
If you're the family organizer and you don't want to share a payment method with your family, turn off purchase sharing. When purchase sharing is turned off, you can continue to share subscriptions like Apple TV+ and Apple Arcade. When Purchase Sharing is turned off, everyone must use their own payment method."
Best regards.