Best place to learn about Network Operating Systems?

I got a free course on cumulus Linux and since we're starting to get whitebox hardware, I am curious if someone could recommend a site or two (or ten) where I could learn everything about the subject.

We have a couple of Edge-Core boxes here and I asked the engineer what O/S is running on them, in which he replied OFDPA.

I guess since i'm so new to all of this, I just want to get an understanding of what does what. For example, is Quagga an O/S? Is ONOS like ONIE? I hear all of these terms and I can't differentiate between them. Rather than google each one separately, I figured if there's a site that exists that explains the more popular ones, that would be awesome.

Thanks all.