Rclone
Rclone syncs your files to cloud storage: Google Drive, S3, Swift, Dropbox, Google Cloud Storage, Azure, Box and many more.
rsync - Samba
Syncthing
Source-code: https://github.com/syncthing/syncthing https://github.com/researchxxl/syncthing-android
Freefilesync
Download FreeFileSync 11.11. FreeFileSync is a free open source data backup software that helps you synchronize files and folders on Windows, Linux and macOS.