There's a blog by a (German?) scholar that covers all kinds of Zettelkastens extensively. Apparently the author is heavily inspired by the method of the german sociologist Niklas Luhmann:

https://takingnotenow.blogspot.com/2007/12/luhmanns-zettelka...

https://takingnotenow.blogspot.com/2016/09/luhmanns-zettelka...

https://takingnotenow.blogspot.com/2016/08/my-translations-o...

And, just for quick reference, his entire 'zettelkasten' tag:

https://takingnotenow.blogspot.com/search/label/Zettelkasten