The place for theory and the basic principles is the “Learn” section. GitHub houses full details of the past and current production versions.
So trying to answer your queries:
What is “this”? emonCMS I think is what you mean. No, it can run on any Linux machine, inside a Linux VM or inside WAMP on a Windows machine.
Feel free to build your own analogue front end. All the basic details can be cribbed from the details of the emonTx V3.4 (the current version of emonTx) or from ‘Learn’. The more care you take over grounding and shielding, the more accurate your low current/low power readings will be. Anecdotally, users who have generally questioned “noise” readings have tended to have ‘breadboard’ front ends.
I’m not familiar with the NRF radios, if you can interface those to send serial character strings or bytes, then you should be able to use them. (If they are interrupt-driven and can only handle one character at a time like the Hope RFM12B, then you might find it hard to run continuous sampling code, but if they have their own transmit buffer, all should be OK.) Much the same goes for a WiFi module. We use the ESP8266 for that.
[Don’t use “emon” without qualification, or we’ll get confused. It’s our generic prefix, so emonTx, emonPi, emonCMS, emonTH etc.]
For info, there are two “Search” facilities here. The one in the light blue header searches just this forum, the one in the dark blue header also searches the old archived forum. There’s a lot more in the old forum, but some attachments have been lost.
They’re a 2.4 GHz half-duplex wireless radio capable of transmitting acknowledged 32 byte packets with a 3 packet buffer, all for under $1 AUD if you buy them in lots of 10. The ones on eBay vary wildly in quality though. I also use them in my home weather station
There’s a great set of Arduino libraries for them from simple peer to peer all the way up to creating Mesh networks.
They don’t provide any tracking of RSSI though which is a feature on the RFM69W in the EmonTx/EmonBase that I really appreciate.