No-one really knows - the wildest theory is that nodes are artificial and as such beyond current means of understanding, while others say its just a very specific natural manifestation of the weakening between subspace and realspace - the latter has its own doomsday proponents that eventually subspace will subsume realspace or cause some other catastrophe, but I'm getting into crazy fictional arguments about fictional lore here and I need to stop myself.

Pretty much every colonized system will have one "path out" - or in, depending on your viewpoint. Two is the norm and Three are relatively regular, the real hub systems occur when four or rarely, five are present, Sol has four for example and still has a fair portion of through traffic. I have my own personal, terrible paint maps of the Soviet Systems and the node lines and spurs they would go into, but its very condensed how I imagine basic in universe maps would be presented for simplicity's sake - like the variation on Earth projections to fit maps - and again its in paint and isn't super useful for anyone but me.

It's still possible to go to stars unlinked by a node line by using a variation on in-system skimming, but that comes down to time and economics which is why its not super common, but not unheard of.

I'll likely pop this post in verbatim somewhere if needs be. Its effectively a system to create strategic and tactical depth in space where typically it can be lacking - theres still some "could come from anywhere" like I say but skimming a fleet poses its own risks and challenges.