Just getting started, basic question

So basically, if I install Roon on my PC or Mac, I can send music from my NAS to Sonos or Oppo 205 wired or wirelessly over my network? This would allow me to play music stored on my NAS in my home theater and also Sonos throughout the house using my PC or Mac as the controller?

Yes. Your PC or Mac houses the Roon Core. The NAS can house the music (or you can store it on an attached drive) and your OPPO/Sonos devices are Roon endpoints.

https://kb.roonlabs.com

Or iOS or Android devices

And the music can be in multiple places as long as sharing is of the folder locations or directly attached drives is workable.

If I buy the Nucleus, that would house the Core and I would just use my Controlling devices to display the libraries and direct the outputs? What is the benefit of Nucleus over installing the Core on a PC/Mac?

It’s really down to personal preference. Some people prefer to have the Roon Core server as just another software application installed on a PC or Mac, alongside all the other applications. Other people take the appliance approach. For example, they would rather buy a dedicated CD player (an appliance) to play their CDs instead of using the CD drive in their PC or Mac, and using a software application to play their CDs.

Nucleus is a Roon Core appliance.

Defer to @Geoff_Coupe on that one. I don’t use ROCK/NUC.