A sort of esoteric (or perhaps just pretentious) question...
First off all, I accept we are a very broad church and we come to commonplacing from all sorts of places. I don't think there's a "right" way to do it and if what you do brings you joy and satisfaction, I'm genuinely happy for you.
I've always enjoyed making lists and taking notes. I would capture snippets and quotations and the like and put them in to one place. And thus, I had a commonplace book. It was something that developed organically from what I was doing; I didn't decide I'm going to start one.
It seems quite a few people now pick some topics then go and research them and write them up. In what was is this different from journalling?
I see some posts from people who want to take up the 'hobby' of having a commonplace book. In my head a common place book just is - it's not something I would guide - I don't know what I want to keep until I find it.
Anyway, thank you for reading. Writing things down helps me get my head around them.