Most of my notebooks are pretty loosy goosy and I add as I need, is a notebook getting a bit full and there's a really obvious division? Time to spin up some subnotebooks 
By far the most complex system is the one I use for keeping my personal and professional journals in, I go into detail here: Anyone use Joplin for journaling? - #18 by Imperial_Squid
Long story short though notebooks for each (ie one personal, one profesh), notes for months, headers for days (though my personal notes are getting a bit long so I'm considering moving to notebooks for months and notes for days but I need a good way to automate that) also have some tricks to auto generate the headers and some naming tricks to make sorting flow better 
