All In app purchases are handled through your default payment method set through either the Google Play Store or the Apple App Store. If you have a default method of payment for purchases on either the Google Play Store (Android) or in the Apple App Store (iOS) that is what is used for purchases.
Adding a payment method on Google Play Store
- Open the Google Play Store app
- Tap Menu > Account > Add payment method
- Follow the onscreen instructions
- The new payment method will be added to your Google account
Adding a payment method on Apple App Store
- Open the Settings app.
- Tap your name.
- Tap Payment & Shipping. You might be asked to sign in with your Apple ID.
- From here, you can change these settings:
- Add a payment method — Tap Add Payment Method.
- Update a payment method — Tap the payment method, then edit your information. You might need to enter the security code that's printed on your credit or debit card. If you use the card with Apple Pay, go to the Wallet app to edit the card information.
- Remove a payment method — Tap Edit, tap the red Delete button, then tap Remove. If you have an unpaid balance, have subscriptions including an iCloud storage plan, or you use purchase sharing, you can't remove all payment methods.