iOS 17.x can no longer login and allow dropbox access on app

The redirect URI functionality works the same way regardless of whether you're debugging or if your app is in development or production status.

If the authorization page is showing the authorization code to the user on the authorization page (with a message like "Enter this code into ... to finish the process") then that means that a redirect URI wasn't used. If that's not what you intended, make sure you're still setting a redirect URI when building the authorization URI.

And yes, when using a redirect URI, after the user authorizes the app, Dropbox will redirect the user's browser to the specified redirect URI with the authorization code included. When using a custom URI scheme (like "myapp://" or "mytestapp://" in your examples), the browser and operating system handle that redirect to send it to the right place. For example, as you said, iOS would see which app has that particular scheme registered and would send that redirect to that app.