How Google Pay opens up a payment sheet as bottom popup (see image bellow) and communicate with the website to notify when payment is done?
Is it a deeplink using scheme such as googlepay://...
or an Intent using iframe as defined here? Or it's something else.
I'm wondering if this kind of popup can be displayed by any application for other purpose.
It is a BottomSheetDialogFragment, and yes you can use it in your application. Read this article for more