Check it out: https://en.wikipedia.org/wiki/Unified_Payments_Interface
Unified Payments Interface (UPI) is an instant real-time payment system developed by National Payments Corporation of India (NPCI). The interface facilitates inter-bank peer-to-peer (P2P) and person-to-merchant (P2M) transactions.[1][2] UPI is an open source application programming interface (API) that runs on top of Immediate Payment Service (IMPS).[3][4] It is regulated by the Reserve Bank of India (RBI) and works by instantly transferring funds between two bank accounts on a mobile platform.
<<
Markets
India
Bhutan
Nepal
Malaysia
Singapore
United Arab Emirates
France
>>Singapore has an equivalent system called PayNow, which is equally instant and ubiquitous.
And when Indian Government declared BankNote Demonetization in 2016, it further boosted UPI because people don't had the cash, so ultimately online transaction is the only option. At that time, E-Wallet also made a hype but it wasn't feasible.
UPI has many advantage for both users and merchants. And further Google also introduced their UPI app Google Pay along with some local players like PhonePe and PayTm which makes UPI transaction easy not just to send money to other be it friends or a merchant but mobile recharge, Bills, insurance, etc. can be done from these apps via UPI.