It's org mode in emacs with the org-roam plugin. If I wasn't using emacs I would probably have a folder full of markdown files instead and leverage a project-based grep to search it. The real power in the system is that it's close to your editor and personal so you can iteratively build up your knowledge.
In particular, every GitHub wiki belonging to an organization or a public project itself is a git repo of markdown files that you can clone and commit to. I probably would use that for per project documentation w/ a separate folder for managing broader notes. I think it's important to separate the markdown from a project's code because otherwise you can have important markdown updates trapped in a branch that hasn't been merged yet and because git works by line rather than by word you can get into complicated merge conflicts by features that touch similar business concepts.
Org mode happens to add some niceties on top in that you can have an example code block (roughly comparable to a triple back-tick code block in markdown) that also can be executed. The example code block can take some arguments like what interpreter to use and will paste the output into another code block underneath the source code block.
Because org mode is also used for organizing tasks it is easy to use it as a scratch pad as you grind through tasks and if something turns out to be useful you can promote it to a top level note (or org-roam node). And even if it's not obvious that it's a note candidate, just thinking through problems 'out loud' in org means you can search and find it later when the same or an adjacent problem comes up.
Another favorite feature I like in org mode is that in any code at any time I can tap out 'SPC n l' and it will capture a reference to the file and the line in the file and allow me to link to it in my notes. It doesn't capture by line number but by copying the literal line, which means that as feature updates push the line number up and down, it still will be able to find exactly the right file in exactly the right line as long as the line's content hasn't changed. It also serves as a rather nice canary because if the docs link to a line that no longer exists, then the documentation has probably gone stale.
The last major win with org mode vs markdown is with org-roam, which borrows the ideas from the standalone program Roam Research (which also is very similar to notion). Every 'node' you make initially is a file and you can fill it in as a wiki type structure. Everything is flat on the filesystem and has a unique id prepended to it so you can't overwrite it with another node of the same name. There's also a UUID associated with each node so if you move from one place to another, none of the links need to be updated. Contrast that with a regular wiki or markdown where when you move a file to a new folder you either have to leave a redirect behind or go through and update all of the back links.
You can also start a node as a subheading and later on promote it to a full file with all the backlinks still working correctly.
I started using org mode years ago with spacemacs, got frustrated with spacemacs, and gave up until earlier this year when I found doom emacs and gave it another spin. I wish I had a better recommendation for an alternative but after looking for 4-5 years for something as good as org mode, I could never find anything. Notion seems like a popular alternative but there's also a lot of shiny bells and whistles in it that can cause 'productivity procrastination' where you spend time configuring and turning knobs instead of actually using the system. Ironically, the second best tool I found was plain pen and paper using a minimal bullet journal technique and taking special care to do indexing. Of course, you can't share plain pen and paper in that way, but the name of the game for personal use is low friction and ready access. The better a system scores on those two in term of note-taking the more useful it will be.
