After you redeem successfully a gift card code, your iTunes balance will be updated with the additional amount that match the gift card.

Please note that from time to time, the iTunes balance doesn't update right away. You can always check the latest balance by following the instructions provided by Apple at: https://support.apple.com/en-us/HT202359

When you have verified that your balance reflect the new amount added on your account, then every time you will be purchasing an app, a game, a movie, a book, then the iTunes Store app will be using the balance credit first for your purchase. If your balance is 0 then iTunes will be using the credit card on file to proceed to the purchase.