Family Share Billing

How iTunes Store purchases are billed - https://support.apple.com/en-us/HT201359 -->

"About Family Sharing purchases

If you're a member of Family Sharing, anything you buy bills directly to the family organizer's account, unless you have gift or store credit. In that case, your store credit is used to pay as much of the bill as possible. If there's a remainder, it bills to the family organizer's payment method.

iTunes Store credit isn't shared with other family members. If the family organizer has a credit on their account and another family member buys something, the cost of that item doesn't deduct from the organizer's iTunes Store credit, but bills the payment method that the organizer has on file.

The family organizer gets the receipts for any of their family member's purchases."

For the Family Organizer:

Payment methods that you can use with your Apple ID - Apple Support - https://support.apple.com/HT202631 --> "Payment methods for Family Sharing

If you're using Family Sharing and you're the family organizer, you can use the following payment methods:

Credit and debit cards

Some additional payment methods available in select countries or regions (Alipay, China UnionPay, mobile phone billing, TrueMoney)"

Bottom Line: If you buy something it gets charged to your balance first, then your card. If somebody else buys something it gets charged to their balance first, then to your card.