On iOS:

1. Open the App Store app.
2. At the top of the screen, tap the sign-in button.
3. Tap Redeem Gift Card or Code. If you don’t see Redeem Gift Card or Code, sign in with your Apple ID.
4. Tap Use Camera and follow the instructions on the screen. If you have trouble redeeming the card, tap Enter Code Manually, then follow the instructions on the screen.
5. Tap Done.

You can find more information in this article.

On Android:

1. Open the Google Play app.
2. At the top right, tap the profile icon.
3. Tap Payments & subscriptions and then Redeem gift code.
4. Enter the code.

You can find more information in this article.