It’s a part of the Roon Server.
You have to differentiate RoonOS and Roon/RoonServer. We are only talking about RoonOS 3.0 here (so far; it looks like an accompanying Roon/RoonServer 3.0 is on its way, too). RoonOS is the operating system on top of which RoonServer runs, just like it would otherwise run on top of Windows, Mac, or Linux.
This probably already tells you what RoonOS provides: the very foundations to make a computer something more than a bunch of silicon and wires, such as being able to boot, hardware drivers, networking, file systems, some kind of system update solution, etc., and the ability to let applications run (like Roon/RoonServer) so that they can do something useful. (It’s called an operating system for a reason)
The last few RoonOS updates were:
RoonOS 1.0 (build 254), Nov 2022
Updated Linux kernel, Updated exFAT and NTFS driver, Installation support for UEFI added, Updated mechanism to switch between Roon branches, Corrected intermittent issues with CD ripping feature
RoonOS 1.0 (build 256), Dec 2022
Fixed an issue preventing HDMI audio output from functioning on some NUC hardware
RoonOS 1.0 (build 258), May 2023
Added IPv6 networking support to RoonOS
RoonOS 1.0 (build 259), Aug 2023, the last update for old installations using BIOS
Service release to make it work with new Single-Sign-on, therefore updates RoonServer too
RoonOS 1.0 (build 261), June 2024, only applies to new UEFI installs since build 254
Resolved HDMI audio output issues on Nucleus TITAN, Improved HDMI audio support for ROCK running on NUC 12 and NUC 13, Added support for Thunderbolt protocol
RoonOS 2.1 (build 271), Sep 2024, only applies to new UEFI installs since build 254
Added Tailscale.
Of course, in a major update they will probably update system stuff to latest versions too, like SAMBA in RoonOS 3.0, the Linux kernel, various system libraries, etc. And it’s possible that they change/add some fundamental things in low level services, like any major Windows, Mac, or Linux update does, but usually the user doesn’t interact directly with this. Though apps that run on top of the OS may be able to use new things and/or expose something in the UI.
Some of it IS mentioned in the EA release notes, e.g., built-in updates for Tailscale or the ability to autorepair damaged exFAT file systems as well as mount damaged NTFS files systems at least read-only. Did you read the release notes? They are here:
SMB, that’s why they use the open source reimplementation of this Windows protocol that is called SAMBA for this very reason.
(NFS is originally a Unix file sharing protocol and is therefore a standard part of generic open source operating systems like Linux. It’s not included in RoonOS, which is a minimal Linux system that has only the parts that it needs to be RoonOS, but it would be trivial to include. It’s not worth adding the complexity, though, SMB/SAMBA is just fine for the purpose and easier to handle for normal users, and if only because Google finds more friendly user-level instructions about setting up SMB shares than it does about NFS)
As before and as usual, the apps (RoonServer in this case) are quite independent of the OS, just like you are currently running RoonServer 2.65 on top of RoonOS 1.0. Of course, it’s possible that future RoonServer versions might start to rely on something new in RoonOS 3.0, 4.0, etc., or that a future RoonOS x.y will have features that you want, but you will see how it goes.