Joplin works pretty well.

It's open source and supports markdown. You also have the option to separately encrypt the notes of you want - although this means they can only be read and edited with a joplin app (clients exist for all OS)

You can install and update it using obtainium. I haven't used it before using it for infinity for lemmy and it works perfectly. Obtainium itself can be installed over android, although I'm not sure it's in the standard repo (I got it from the IzzyOnDroid repo)