The payment doesn't happen on the device. It happens at the VISA/Mastercard/AMEX level. So "the app" should be doing the validation upstream on the server side

The payment doesn't happen on the device. It happens at the VISA/Mastercard/AMEX level. So "the app" should be doing the validation upstream on the server side and simply reporting yes/no to the app. If you hack the yes/no okay but most payments are for physical things IRL so the payment gateway that is not on your phone is gonna be like ??? - in other words it's on the app not to trust a raw true/false signal and instead rely on server side checks. This is all irrelevant. I make regular massive purchases on my linux box where I can read the memory. It doesn't change anything.