Where/How to learn more on Network Operating Systems?

Hi all,

All the apologies out of the way first. Apologies if this has been answered before but my Googlefu is particularly weak this evening. Apologies if there is a subreddit that is better suited to this question.

My question centres around how the Network Operating Systems are built. The IOS, JunOS, EOS so on and so forth. Now, I am thinking it is heavily down the computer science path and similar to programming any OS, Windows, Mac, Android etc. However I haven't been able to find definitive answers and my online searches take me from ARPANET to Novell to MacOS (none of them have been thorough either).

Is there any specific site or really, anything? I'll take whitepapers or copies of old NOS code at this point. I am keen to learn more but I don't really know where to start.

Thanks

TL;DR - I like Network Operating Systems where can I learn more?