FedNow is infrastructure for settling payments between banks. It's replacing ACH, which is really old and slow to settle.
In computing terms, think of FedNow as something like TCP: it's very low in the OSI hierarchy and not meant to be used directly by users. The hope is that it will enable nicer applications to be built on top of it, things which are not really possible to build on top of ACH, but it is not a payment application in and of itself.
Well, there is Real Time Payments, as it is called RTP by The Clearing House. (RTP is almost like FedNow.) However, The Clearing House is owned by a small number of banks. That's why only big banks use RTP. I heard about 50 banks use RTP, when 10000 financial institutions exist in US. This is also one of the reasons why majority of credit unions don't offer Zelle, as the latter is owned by top banks.
The bigger culprit is private entities controlling infrastructure, and these entities want their tax.
In computing terms, think of FedNow as something like TCP: it's very low in the OSI hierarchy and not meant to be used directly by users. The hope is that it will enable nicer applications to be built on top of it, things which are not really possible to build on top of ACH, but it is not a payment application in and of itself.