As global regulators increase their scrutiny on Visa and Mastercard over merchant fees, India has chosen a different path: creating rival payment networks that are increasingly sidelining international card networks. The strategy builds on India’s Unified Payments Interface, known as UPI, a nine-year-old system that now processes more than 13 billion real-time transactions monthly, or about 71% of all transactions in the world’s most populous nation. This payment system, which lets consumers and...