A UPI QR code and soundbox is how most Indian shops accept UPI payments: a printed QR code on the counter, and a small speaker that announces each payment aloud. The speaker is called a soundbox, or a payment speaker.
The QR code is a printed picture of the shop's UPI ID. A UPI ID is a payment address, like an email address, that is linked to the shop's bank account. The shop needs no machine, no electricity and no internet to display the QR code. A QR code generator creates it.
Here is a typical case. You buy a dozen bananas for ₹60 from a fruit seller at a weekly market in Mysuru. You scan the seller's QR code with your UPI app. The app reads the seller's UPI ID. You type ₹60 and enter your UPI PIN. Your bank debits ₹60 from your account. UPI, the public system run by NPCI, sends the payment to the seller's bank. The seller's bank credits ₹60 to the seller's account. Inside each bank, a UPI switch receives the payment message and passes it on. All of this takes a few seconds.
The fruit seller is busy weighing fruit and cannot check a phone after every payment. So when the payment arrives, the seller's payment company sends a message to the soundbox, over a mobile SIM inside it. The soundbox then says aloud, "Received ₹60". A customer cannot fake this announcement, in the way some people fake a payment screenshot.
Most UPI payments to shops carry no fee. So the payment companies earn money in other ways. They charge the shop a monthly rent for the soundbox. They also offer the shop loans, based on its daily UPI sales. On the customer's side, a credit line on UPI lets you pay the same QR code with borrowed money, instead of money from your savings account.