Field Notes notebook and a Staedtler Triplus Fineliner, in my pocket at all times.
Remarkably, I find that the Field Notes slogan is true: "I'm not writing it down to remember it later. I'm writing it down to remember it now." I rarely consult my notes after I write them, and memory written as tactile sticks a lot more than typed notes.
Occasionally I'll use the stock "notes" app but that's really temp storage for the most part, then anything I do with those notes becomes the thing to which I refer back.
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