I use syncthing across various devices and organize a group of folders with markdown files in them, then view / edit them with a markdown editor.
Super simple, free, and the best note system I've used so far.
I directly take notes online. See here: learnook.surge.sh
Then in the chapter 2019.
I take a lot of notes the old fashioned way because I find it helps my retention, but they can pile up fast when I'm particularly busy.