Use assets/applepay.json as your paymentConfigurationAsset for Apple Pay. You can save the file path to a variable the reference it in the paymentConfigurationAsset field. Do the same for google pay . Here is is referenced in the docs you linked https://github.com/google-pay/flutter-plugin/blob/main/pay/example/lib/payment_configurations.dart

ApplePayButton(
              paymentConfigurationAsset:PaymentConfiguration.fromJsonString(
 applepayJsonReferenceToAssets)