Currently this works between banks only. Banks have reserves at the Fed, and those reserves are what gets adjusted when a FedNow transaction happens. The banks then adjust their own records on their customers' behalf.
The fed lists a number of service providers on their page. I assume they will now roll out some interfaces. Glad to see the US banking system start to catch up with the rest of the world ca mid 90s.
It won't take long before all banks are using it. Once the banks are using it, employers will begin making payments using it. Once employees are receiving digital currency payments from employers, cash will end soon after.