Is there any way to manually save a local note in Joplin, or does the app just perform automatic saves at regular intervals? Is it the same for the Android app?
My line here is very poor, and sometimes I want to hit a "save" button just to be sure! Though synced to Dropbox target I occasionally notice minor discrepancies between my mobile and desktop notes.
Changes are saved automatically. This is true both on desktop, and on mobile. When it comes to synchronisation, local changes are also automatically uploaded after a short delay. However, you should be more careful on Android with this, as if you close the app immediately after editing a note, it may not have enough time to sync the last changes, and because there is no background sychronisation, the changes will only sync after you open the app again manually. The same is true for the desktop version, but here the default is to stay running in background, so it should still sync even if you "close" (or actually minimise) it.
As @tomasz86, says syncing is automatic, but you can manually sync if you've just done a lot of editing and want to be sure you don't lose anything. I occasionally do this, but have generally found Joplin to do a great job of taking care of it without intervention; I've good confidence in it after years of use.
Ways to force a sync (in effect "Save"):
The key shortcut Ctrl-S
Click on the Synchronise label in the Note Book panel
Click Synchronise in the File Menu
Or select Synchronise from the Command Palette (Ctrl-Shift-P)
I'm fine with them being saved in Joplin (my Dropbox target, actually). I just wanted to be sure that changes to a note have actually been acknowledged and saved by the app.