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. replies(1):