Need help understanding how syncing works

I’m currently running Syncthing on both my Mac mini and in a docker container on my unRAID system. I use it to back up my Plex library. I have Plex running on my Mac and the “master” library (the library Plex pulls the media from) is a 5TB USB HD connected to the Mac. The backup is on the unRAID system. I also have Sonarr and Radarr running in dockers on the unRAID; currently they’re set to move any downloads to the unRAID backup, which would then copy them down to the master during a sync. While watching it do its thing, it got me wondering about how syncing works in Syncthing.

Does Syncthing use the concept of a master/backup arrangement? If so, is the “master” whichever system Syncthing is installed on? Do I need to be running it on both systems, or will one suffice? I recently had to delete some media from the USB drive, and during a recent sync, I discovered that Syncthing put those deleted files right back on the drive from the backup. I don’t have any issues with this, I just want to better understand how the two systems decide which files get changed.

Thanks

There are some writings on it;

https://docs.syncthing.net/users/syncing.html

I read that article before posting but it doesn’t really answer my questions about running two instances on different systems or the relationship between the two systems. If I’m running Syncthing on System A with a folder of files I wish to back up onto System B, does that make System A the “master”? Do any changes made outside Syncthing to the backup on System B get undone when Syncthing does a scan against its master folder on System A? If two systems are running Syncthing and pointed at each other for the backup, does that mean each system sees itself as the master?

There is no “master” in Syncthing. All devices are equal. Also, Syncthing’s main purpose is two-way continuous synchronisation. You can, of course, use it as a part of a wider backup system, but Syncthing by itself is not a proper backup solution.