Zkn, Zettelkasten, is popular note taking invention by Niklas Luhmann. It is personal database of sorts comprised of physical boxes, paper Cards.
Niklas himself only gave hints about the system but did not explain it full, other than the fact if anyone to open it will not find anything useful. The reason for this is important cog in the system is Niklas himself, he is the key to unlock the machinery, without him it will be bunch of papers, even if you figure it out all the interconnections. >> zkn zettelkasten
There are various books and articles tried to explain this but in my opinion failed and also made it worse with some kind of trivialisations with all good intentions to make it some kind of system with principles. Or some of the articles explained it with what is at hand, which is actually nothing without Niklas himself. I think it is a database and what we see is database tables and columns arranged by Niklas and actual data which is knowledge possessed by Niklas. The physical medium is just bread crumbs used by him to get to the idea he have in his mind.
Now if Niklas wants to write about Zkn as methodology in hypothetical scenario, he will traverse this database tables and columns made up of boxes and note cards to come up with relevant query, he keeps traversing the path or bread crumb left by him in note cards, he would know he has written something about zkn in 2a3b1c, looking at the card he find some more bread crumbs, the information connects to 3 more note cards, but only one can actually be related to Zkn as methodology, so after finding that surprise connection he goes to pick the new card, so the story goes till Nik is satisfied with all the related stuff he needed, he also discovers, some more ideas something much better than writing Zkn as methodology, something social scientists would be interested in, so he abandons to write about Zkn as methodology goes back to writing awesome new book about Social Structure. With all the apologies to Niklas Luhmann with no disrespect intended, this is what makes up Zkn an extension of mind for Niklas but anyone else trying to get information or a system out of it to fail. The system is both trivial and extraordinary, it is trivial because it is fragment of interesting stuff with links to relevant stuff, it can have line-wise connection like linked list but also branch out to form new connections, and also connect to something not in the branch of thoughts group it was originally conceived spanning many chains of thoughts. On the other hand it is a masterpiece because it is the garden of knowledge with extraordinary connections and collections over the decades, it is massive in scale and achievement.
To sum it up, it is bread crumbs of information after information, connected not with random number, but person who understands the working context of both the note cards and how they link to each other. The connection is important is also the person who is doing it. These chains of thoughts usually don't get to any centers like video games, these is no actual centers as such, the Center, if there is such a thing will come from seeker and the creator of the system having specific need or query and seeking to find answers, instead of looking at every note he ever created every time he was looking for something, he created meta cards and other groupings, still for the volume he created he still have to go through so many, but this only resulted him finding new stuff what we can call Eureka moment leading him to surprise knowledge or discovery. IMHO this is something anyone should be able to do, but takes diligence and years of practice to understand the stuff one creates physically or digitally, find relation to previously created information, create that relationship in both the notes or as many notes as related, keep doing it for years to come. So when you go back to the notes it is richly connected by you with you knowing each card you can traverse many directions as your connection allows discover surprising hidden notes or cards that you have long forgotten, serving you just like Zkn served Niklas Luhmann. The idea is both simple and monumental but without years of practice at keeping the system, one cannot acquire any knowledge out of trivialised principles or tools geared towards Zkn either digitally or physically.