UPI is a product of National Payments Corporation of India (NPCI), a company promoted by RBI and Indian Banks' Association (IBA).
The idea is to creating a robust Payment & Settlement Infrastructure in India. A part of this area is currently dominated by foreign companies i.e. Visa, Master card.
The company is 'Net for Profit' company. It charges from bank for payments and inter-bank settlements. Eventually they may start charging a small fee from retail customers also.
You can find more information about: https://www.npci.org.in/product-overview/upi-product-overview