NDX2 will be the streaming endpoint. It is the destination of the stream and therefore uses the network interface as well as the DAC. You would use a roon remote (e.g. on an iPhone) to control the streamer.
Roon core, e.g. a NUC or a Notebook would be the streaming „starting point“. This is oversimplyfied but may help. NDX2 is a good roon endpoint and makes good use of the NDX capabilities. You simply do not use the Naim app for controlling your music replay but a roon app.
Some Roon Core devices have a USB output that could be connected directly to a USB DAC, and that would work perfectly well, although in line with Roon’s recommendation, a network connection is potentially better and more versatile. In this case the Roon Core is acting as a server, and the Roon ‘Endpoint’ is a streamer, not just a DAC.
You can think of it that way if you prefer, but the NDX2 will be using its streaming board to accept a TCP/IP stream in much the same way whether it comes from a UPnP server, a Tidal or Qobuz server, or a Roon Core. That implementation of the Roon Core, using RAAT instead of UPnP, makes it a server in my books.
I thought you meant the USB connection, but what I meant to say is that the roon core is the device that will pull the data from either the streaming service on the Internet or the local NAS, and then send it to the endpoint (s), always.
Yes, Roon recommend that you use a network connection to keep the electrically noisy Core hardware away from the hifi. Besides, a USB connection wouldn’t work into an NDX2; if your Roon Core has a USB output you would need to convert it to SPDIF.
Given that both devices need a network connection in any case I can’t see any reason to add an additional direct connection that you don’t really need.
Roon and a Naim streamer work very well together in my experience.
Using Ethernet cables, between Core and Endpoint, when you stream from say Qobuz the stream goes to the Core, where it is processed to PCM and PCM (or DSD) is sent to the endpoint I.e. Naim Streamer.
This puts less work on streamer than using Qobuz in Naim app.