The merchant's system displays this code, you open your online banking app, scan the code, select "SEPA INST" (here's the usability catch!) to make the payment instantaneous, and confirm. Within 10 seconds, the money is transferred to the merchant's account. Either the merchant's bank or a third-party Open Banking API immediately informs the merchant's system (e.g. by push notification or webhook), and a receipt is issued.
Everything is already here, but since this system would be virtually free to use, nobody really has an incentive to push it. It costs money to educate the public, and there is no money to be made. Instead, everyone gets paid handsomely by the card mafia.