Syncthing
Released in 2013, Syncthing is a self-hosting service for synchronizing files and folders between devices. It can be used over the Internet or a LAN (local area network) to share files with multiple users or different devices owned by the same user. When a folder is shared on Syncthing, any files created, deleted, moved, or edited are also done on all connected devices in real-time.
Syncthing is free and open-source and can be downloaded on Windows, macOS, Linux, BSD (Berkeley Software Distribution), and Solaris computers. An Android app is also available on the Google Play store.
Can I use Syncthing on an iPhone or iPad?
The original version of Syncthing does not support iOS and iPadOS devices. The developers have stated that there are no current plans to release a version of Syncthing for these platforms. However, since Syncthing is open source, there are several forks in development that add support for iOS devices.
If you want official iOS support, other services like Nextcloud can self-host a file-sharing server.