Banks implement the UPI protocol (an API essentially) to talk to the NPCI infrastructure network. Your UPI VPA is linked to your bank account. Most banks in India are already on the NPCI/UPI network.

Thirdparty apps can also implement the API to provide services on top of UPI eg: bill pay, ticket booking for buses, trains, movies etc.,

Payment processors provide UPI as a payment method (in addition to debit cards/credit cards/netbanking/digital wallets etc.,).

To answer your question on credit cards usage with UPI: No, you only need a bank account. UPI is a p2p payment overlay over the banking network.

Credit Cards on UPI is planned, but pending approval from the central bank: https://indianexpress.com/article/business/banking-and-finan...

As it stands, it's common infra for a lot of instant settlements in the country, including payments for toll-booths at highways.