Think standard oAuth. Github has multiple flows that should cover most purposes here: https://docs.github.com/en/apps/oauth-apps/building-oauth-ap.... The key is that payment data is encoded in oAuth-like scopes, so all authorizations are scoped by amount and lifetime, and are implicitly merchant-specific.

Browser-based flow, where you're already logged into the bank in an existing browser tab:

* Amazon redirects you to oauth-proxy.visa.com where you select your bank (if you've done it already once, it remembers and redirects straight to your bank)

* Visa redirects you to your bank - if you're not logged in, you do a login - this is up to your bank on how to do that - authorize with an existing phone, WebAuthn, etc. On OSes supporting it, this URL can be hooked and handled directly by a native app which may use the device's secure element to store its auth credentials for the bank

* Bank displays you the payment request details (which include your Amazon account email, order ID, etc - all info you need to confirm it's indeed your payment request and not someone else's) and allows you to change them (maybe you want to authorize more or less, or make it one-time/recurring with a daily/weekly/monthly/yearly cap, or set an expiry after which the authorization is no longer valid)

* In the background, Amazon gets a success webhook from Visa (or their processor) saying that this authorization request has been granted, or they can poll an endpoint - this eliminates the need for a final redirect back to them like in normal oAuth

* If this is a recurring charge scenario, Amazon can store this payment request token against your account and use it multiple times, as long as the charges fall within the policy set during initial payment request establishment (if you set a max of $20, they can do as many transactions as they want up to a total of $20).

Device-based flow, where you aren't/don't want to login in to the bank the same browser:

* Amazon redirects to oauth.visa.com as above

* Instead of clicking on your bank directly, you say "authorize via phone", it just encodes the URL of the current page in a QR code so you can scan it on the phone - you then do the above flow there. Because the success/failure of a payment request is already communicated directly between the merchant and Visa, there is no need for your phone to pass any data back to the browser, so no need for a "reverse channel" to be set up.

* On your phone, you may have your banking app installed, so it takes over the domain name of your bank and automatically opens the payment request authorization there, using your existing session within the app.

Point is, not only is there no longer a concept of a card number that can be copied, stolen, or leaked, but the user also remains in control - they can control whether the payment is one-time or recurring, set limits on recurring payments, and be able to cancel these authorizations at any time, after which they're guaranteed that nobody can take more money without going through this auth process again. This eliminates many reasons for chargebacks, and reduces fraud risks for merchants too (merchants are no longer vulnerable since the auth to authorize a new payment request is between the user and their bank directly), so things like behavioral fraud detection or captchas on payment pages are no longer needed.

Downside (for scammers): business models based on a free trial that rely on the user forgetting to cancel, or those who intentionally make cancellation annoying or impossible wouldn't work, because payment requests should list upfront the max amount they can take, and the user can adjust that and make sure the unwanted charge just won't go through even if they tried.