Have you tried actually using Joplin
? The notes are stored inside its database. In other words, there are no external files to edit, and you cannot use Joplin to open any external files either. Of course, you can create a new note and copy and paste text content from external files inside it.
By "locally" I mean that Joplin operates fully offline and stores everything on the disk. This doesn't mean though that you can access the notes as files, etc. using other software. You still need to open Joplin and use it to manage the notes, which are then stored in the Joplin's database.
I'm not sure if I can explain this in more simple terms
.
No, Joplin has nothing to do with neither Git nor GitHub (except that the source code is stored there), so no such synchronisation is possible.