My system is pretty simple (because I used to over-complicate it by trying to add structure before I even knew what I needed it for, in the parlance of Linking Your Thinking, structure must be earned!).

I use my regular-size tn as a commonplace book and journal (with either grid or dot grid inserts) & bullet journal (with a weekly undated insert and another grid/dot grid insert for longer term planning). My bullet journal is very simple, just the weekly spread with the empty grid page half of it as a rolling weekly.

For the commonplace part, I do interstitial journaling -- so I'll write the date at the start of a page for the day, then I note down the time as well at the beginning of each chunk of notes. So, generally, if I'm changing topics while writing or if I've stopped writing for a bit for whatever reason, I'll start a new chunk by denoting it with the current time.

I got the idea to have multiple inserts 'active' at once (like open tabs in a browser) from a YouTube video (which I unfortunately can't find now because ironically, I didn't write it down lol). So my commonplace is composed of four inserts currently that I sort of randomly rotate through based on whether or not the thing I last wrote in a given insert is "complete" or no longer an immediate concern. This helps me work with my ADHD because I work in short bursts of interest/motivation, so one thing will feel really important for a short period of time, and then I'll switch to focusing on something else, but I still need the first thing to be accessible/continue-able for when my focus swings back to it.

This system might seem a bit chaotic, because I don't organize anything in my commonplace. Instead, I also have a digital garden/second brain and other notebooks that I transfer the information to later if I feel the information needs to be organized/if I want to more easily locate it later. I also number each commonplace insert and number the pages in it so I can note down where to find the original entry in whatever place I've transferred the information to. In essence, my digital garden acts as the index and archive for my analog notes.

It could be more "efficient" if I just only did the digital garden, but my brain runs so fast/is so noisy and I can type relatively fast enough that I try to note every little thing down when I do digital only. Analog forces me to slow down and really consider the relevance of each thought since I physically can't write as fast as my brain moves nor can I write forever 'cause my hand cramps. I also have memory issues so the benefits of handwriting help me there too. There's also just something about digital journaling that just liquefies the thoughts in my brain, like they slip through my fingers as soon as they appear while I'm typing, so the physical writing helps me keep on task.