Me, all the time. as @tf2 says it’s all the things and ideas and notes.
I guess I disagree that those are “carefully curated” that may be the ideal but is sure as heck isn’t the reality of people actually using those systems vs those trying to sell you on the concept. The sellers have carefully crafted incarnations that look beautiful but may not work in practice. It’s up tot eh user to figure out what works.
I might be able to find that info by searching but I can find it faster with less angst by refering to my note on gardens where I linked in the name of the person and what they suggested we do about some large trees sometime in the next 3-5 years.
Me too. My first was spiral bound notebook with divided sections, once for each class and one or 2 for my personal ideas and fun stuff. I typically had 2 or 3 of the 5 or 7 sectionnotebooks at any given time so ech area was somewhat separated. I added a paper calendar to that. Then Covey style planners, DayTimers, 3x5 cards ala Slob Sisters, paper checklists laminated so I could wipe them off and reuse them and more. Now it’s Obsidian, Omnifocus, Apple Mail, Apple Calendar and an extensive paper and digital filing cabinet structure. The current version is much easier to use and locate stuff in compared to the old one and I’m slowly scanning and making digital all the paper I can.
My Obsidian vault is called my Adversaria as a nod to the commonplace book.
You could argue that pretty much all of civilization, writing, story telling, pretty mcuh everything external to us is a second brain of sorts.
And in some ways that came out of Heinlein’s “Beyond This Horizon” originally published in 1942 under one of his pseudonyms.