Good question! Wish I knew the answer!!
I’m a longtime credit card user and have no use for UPI but I tried a couple of UPI apps just for the heck of it.
Soon after installing an UPI app, and tapping a few buttons, I was presented with a long list of my bank accounts. I assume they were fetched automatically by the UPI app on the basis of their respective registered mobile phone numbers, which is my one and only mobile phone number.
After I tapped one of them, I was asked to set up a UPI PIN for it, which I did. I then saw this account labeled with UPI ID xyz@abc, where xyz was a proxy for my name and abc was a proxy for a bank.
When I installed another UPI app and selected the same bank account, I expected that it would be represented by the same UPI ID xyz@abc because, according to the popular narrative, UPI ID is an ID for a given bank account.
But that was not to be. The second app created another UPI ID for the same bank account. I found this to be terribly disconcerting and abandoned my exploration of UPI.
So, in a nutshell, I don’t have an answer to your question. Best wishes to you for finding it.