I think maybe there’s a misunderstanding here.
You have two clients. The QT app (GUI), and the daemon (headless).
The daemon (nud) runs as an RPC server by default, so you can send commands to it.
The QT (nu) does not have the feature enabled by default. In order to enable RPC commands against the QT app you must add server=1 to your conf file. Then you can run RPC commands against it like you do with the daemon.
I’m not sure why @cryptog is setting the RPC port in his conf file unless he wanted to use a port number besides the default.
You can see all of the default port numbers here: https://docs.nubits.com/rpc-api/