Does Syncthing not sync files to devices it deems is not approved?

If you had 3 devices which are…

Computer 1 Computer 2 Phone

And the following devices are synced with the same folder synced…

Computer 1 - Phone Phone - Computer 2

If a file is created/edited/deleted on Computer 1, will it every get synced to Computer 2 without having Computer 1 and Computer 2 synced? Or does Synching block files from being synced from one device to another if the two devices are not setup as synced devices?

You don’t need to connect all devices with one another for synchronisation to work. Just beware of conflicts if you edit the same file on Computer 1 and Computer 2 while the phone is switched off or disconnected. In other words, I’d make sure the middle device is turned on and always available.

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Put differently, yes, Syncthing only talks to the devices it explicitly knows about. But given A <-> B <-> C, A syncs with B and B syncs with C, so stuff from A will end up on C eventually.

The reason I ask this is because I am syncing my Joplin notes. And out of the 3 devices, one of them is usually turned off for a good few weeks and I am not sure if this is a Syncthing issue or a Joplin issue were the notes syncing will break and then devices will not be up to date or sync over time.

There’s a known problem in Joplin regarding file system sync (see https://github.com/laurent22/joplin/issues/6517). Essentially, unless you apply a workaround (such as the one that I recommended there), file system sync is bound to break at some point, and Joplin will fail to add the synced notes to its database. I say it’s bound to break, because the breakage will happen as soon as you open Joplin on the device before Syncthing file synchronisation has taken place yet, or in between the synchronisation, which is very common, e.g. on boot and user logon, or when resuming the device from sleep and such.