RTP® is an instant payment service rail from The Clearing House; it is completely separate from the FedNow Service®, which is an instant payment service rail from the Federal Reserve Bank.
Instant payment services transfer funds between participants immediately via a credit-push only payment rail.
Though RTP does stand for Real Time Payments, it is The Clearing House's service name and does not encompass all instant payment services. RTP is a real time payment solution, but not all real time payment solutions are RTP. (Think about it like rectangles (real time payments) and squares (RTP). All squares are rectangles, but not all rectangles are squares.)
RTP and the FedNow Service are not applications. They are simply communication conduits (only payment rails). Whoever you use to connect to these rails (CU*Answers, Corporate One, etc.) needs to build the infrastructure to initiate/receive the communication.