I am developing an application and I want to add a payment gateway to sell certain services. So far I have seen several gateways such as Stripe or Square, but this is not what I am looking for. My intention is that the typical "Pay with Google" button or something similar appears and that it takes you to google play where the user has already entered his credit card, etc. The problem is that I don't know where to find information about it, as much as I search I can't find anything and I don't understand how the information can be so hidden being something so common.
I do not pretend that you tell me how to do it, if not knowing where to look for information about it and how to start.
Stripe supports Google Pay: https://stripe.com/docs/google-pay
There are also Stripe plugins for Flutter and Dart, if you prefer to use that integration directly.
https://pub.dev/packages/stripe_payment
https://pub.dev/packages/stripe_sdk