I have been spending some time on the personal knowledge base portion for a little bit. I currently store my notes in flat markdown files that get linked together by some local wiki software called vimwiki.
What I like about vimwiki is that it is built into my favorite editor, can be configured to use markdown, and is 100% local. This is ideal as I have machines with bad network connectivity. With iSH in the App Store, I am able to use it on all my devices (vim works really well on a mobile phone, believe it for not). I keep the files in sync by committing them to a private GitHub repo. I am currently working on adding a good backup solution for all my GitHub repos, but for now it is a good fit and ensure’s good conflict resolution and decentralization.
I manage all the config in a dotfiles repo which ensures I have a consistent setup across devices.
The older I get, plain text seems to be the most secure format for saving/archiving information. These tools let me do they and store information I learn or generate. The specific tools don’t matter as long as you can find something that sticks and you keep using it.