I carry a small Muji passport notebook everywhere. It fits in my pocket, and is super thin so it opens flat. It's the one notebook where I capture everything with no set routine. It's random. I finish these notebooks fast. 5-6 in one year in addition to my other notebooks. I use it for a couple of things.
First is just plain and simple capturing. An idea, a quote, a line of thought, few random topics that I should learn about, a blog idea, or the name of a brand I should check out later. In the notebook, it's 1-2 lines per idea. No categorisation or structuring. At the end of the week, or a few weeks, I go through it. Tasks move to my Reminders app. Ideas move to my Notes app. Everything else that I act on, I cross them and eventually, that notebook's job gets done. That notebook is basically my temporary capture zone.
Second is the occasional daily log. Five to six bullets a day about wha actually happened. Small facts like 'Met X.' 'Finished writing Y' 'Ordered soup.' Just enough to remember the day later. I don't refer to them much later but as I keep stacking my notebooks, maybe I will look back after a few years. This is my method of creating memory anchors for the future. Sometimes, it becomes a method to see what repeats. For instance, what gives energy, what drained it? Sometimes, I just use this to plan the next week.
Lastly, it's a small brain library. I write lines from books, some notable talks, quotes, or names of people that changed how I think. Basically, when something hits me, I write it by hand and add a few sentences on why it matters. That's all.
In some ways, the first one clears my head. The second one loosely records my life, and the last one influences how I think. Good value add for spending about 10-15 mins daily in different pockets of time.
The notebook is where I think. Writing forces friction which in turn builds clarity. The respective apps like Reminders or Notes are where I store. You can search anything so it's great for retrieval. I use a Muji passport notebook which is small enough to carry everywhere and use without planning. That's why it works.