Explain what actually happens in the two seconds...

Explain what actually happens in the two seconds after someone taps pay on a UPI app. The audience is a curious customer who has used UPI a thousand times and never once wondered how it works. Open on the tap and the sound of the confirmation. Then follow the request through the app, the payment network, and both banks, showing the check that has to pass at each step. Turn on the part nobody expects, that the money moves in a different flow from the confirmation you just saw. Land on why the whole thing still finishes before you can put the phone back in your pocket.