I can second the recommendations i've read here. My approach to knowledge management has been super simple. Everything is in git, in a big directory structure i've organically grown over time. I have so many notes.. 7 to 8 thousands of them. Keeping it in git has allowed me flexibility to use it how i see fit. Sometimes i'm in emacs/nevom. Most the time i'm in sublime text because I have workspaces per project/concep…
For iOS, I now use Working Copy for git commit/push/pull, and Obsidian to edit text, and I like Obsidian's interface a lot for making quick Markdown edits.
I believe Obsidian has an Android version as well, but I have not used it. I'm not sure if there is an Android version for Working Copy but there may be similar GUI git apps.