Being able to send money to other people easily is a huge part of living in a digital world. We may not trust FB with our payment data, but millions of people probably will.
I leave mine on Public too.
UPI is Unified Payment Interface; sort of RSS but two way, anybody can make client for it, UPI never holds money, Clients connect to user's bank accounts (any bank); client send & receive money to/from bank accounts only, but UPI streamlines this, its kind of hub, but money flows direct between two accounts.
Most banks offers their UPI clients in their main app. A user need a bank account & then he can either signup on his bank app like SBI HDFC AXIS or any common app like Google Pay, Phone Pay. The username is like email address, user@bank style. Only banks offer the part after @. Amazon uses @abl, phonepe uses @ybl.
Once you type in user@bank in any client, it fetches the current official name of user. UPI does not hold the info, but gets it in real time from @bank.