I've tried a lot of different systems myself. At work I use a combination of daily checklist that I created for myself and the monthly momentum planner worksheet from productive flourishing. https://www.productiveflourishing.com/free-planners/ with blank notebook paper for meeting notes. Work stuff stays in my office, so I keep that totally separate. I have a sketchbook and a small notebook that I use to keep notes about video games that I am playing. So no, sometimes "one notebook for every purpose" is not realistic. The little bullet journal is just my everyday carry thing, sometimes things that go into it get transferred elsewhere once I have access to the correct system.