All purchases are billed to the payment method that you designate when you sign up for an Apple ID, unless you have gift cards or content credits.
Depending on the payment method that you use for a transaction, Apple might obtain preapproval for an amount up to the total price of the order. Your order might also be authorized and billed in increments during one purchasing session, so it might appear as multiple orders on your billing statement. Learn about the payment methods that you can use in the iTunes Store, App Store, and iBooks Store.
If you have gift cards or content credits
If you use a gift card for a transaction, the amount is deducted at the time of your transaction.
When you make a purchase, content credits are used first, as long as the value of the item that you're buying is less than or equal to the value of the credits. Any gift cards are used next. If your gift card is less than the total cost of the purchase, the payment method that you have on file is billed for the remaining balance.
If you have no content credits or gift cards, the payment method that you have on file is charged for the entire purchase.
When you gift an item to someone else, no credits are used. The payment method that you have on file is billed for the entire purchase.
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.