I used to have an elaborate system, but I converged on a simple solution: I stash everything in a single Google Docs document.
I made the conscious decision to optimize for ease of use, so that the friction/effort to write something down is minimized.
At the same time, I also made a decision to not to adopt any organization system -- anything that increases the friction of use is eschewed. Search was all I needed. I've been using this system for the past 5 years or so, and it's been very productive.
To extract ideas from it, I routinely re-read stuff (it's in log format, so it's very easy to read) and use the Fieldstone approach (Weinberg)[1] to coalesce similar and interesting thoughts and rewrite into larger thoughts. I've gotten a lot of actionable ideas this way (that I actually go on to execute on).
So it's a system optimized for postprocessing rather than pre-processing. (The Fieldstone approach is a method from writing, rather than knowledge management.) I find that pre-processing systems are unsustainable over the long term unless you're exceptionally disciplined (also, it's hard to know how to structure knowledge until you've processed it -- most interesting ideas are a garbled mess when first encountered). I try to build systems that don't rely on sustained human discipline or the necessity of shoehorning into known organization units.
[1] https://www.skmurphy.com/blog/2016/05/04/weinberg-on-writing...