The BCB in Brazil does very little to operate Pix. It's effectively a P2P system, where the BCB forces all the banks to interop with one-another (and all the banks directly call eachother). They can operate it that cheaply because they do close to nothing technically (they host the main discovery endopints). The only place the BCB actually ingests data is via their reporting mechanisms.

UPI is a bit more centralized, where the NPCI does the top-level routing between banks, so their operating budget is likely much higher than Pix. It also is drastically more simple to be a participant in UPI compared to Pix.

For Pix adoption: you can thank Covid for that. The Brazilian government said if you wanted to get free money from the government, you had to set up and use Pix.

US Financial Innovation: I'd say the hard thing here is that the government is extremely strict (lots of regulation) when you start looking like a bank. Lots of companies have tried to innovate here, but regulation makes it really hard to do. There's a lot of regulator capture going on.