>     I've seen syncthing is available on Devuan apt repository. On F-
> Droid, I see three applications: BasicSync, Syncthing-Fork, and
> Syncthing-Lite. Do you recommend one?

I have 'Syncthing' on Devuan machine and 'Syncthing-Fork' on my Android
mobile phone. They speak to each other with no issues.

>     Other question: Does it synchronize one directory on each side, or
> everything, or what else?

Syncthing can sync multiple folder sets over multiple devices.

For my example, I have one sync setup so '/DKIM/Camera' folder on my
Android phone syncs with '/home/nick/camera-photos' on my devuan
machine. I have it set to sync both ways and only over WiFi to save
data. So every time my phone is on WiFi it copies recent camera files to
my home machine. Any unwanted photos I delete on my home machine are
then also removed from my phone. Sync is instantaneous and in the
background, ie you don't need to trigger it. I also have another folder
'Docs/shared' on my phone and my partner's phone that syncs to
'home/nick/shared' on my machine and 'home/partner/shared' on their
machine for other stuff, eg copies of home insurance policy.

Syncthing has lots of customisation if you need it, eg ignore some file
types, sync one-way, etc.

Worth repeating Syncthing is not a backup. If you accidentally delete
all files on one device it will automatically delete them from all
linked devices.