Comment by sandreas

As reference (for feature comparison or whatever) here are some tools I use to keep things in sync, some have already been mentioned:

  rsync - I think everybody knows this
  rclone - modern feature packed tool similar to rsync, but also more complex
  croc - a modern cross platform file transfer utility
  syncthing - sync service with web gui to keep multiple devices in sync
  LocalSend - An open source GUI tool also for mobile devices
  restic - a tool for encrypted backups with rolling hash deduplication
  immich / ente.io - Photo backup app with frontend and backend
  zfs - filesystem with send and receive

All of these are (partly) open source and free to use. Hope it helps.