Let's say you want to backup some data on remote server.
Easiest option is to use rsync.
On the remote server you will need account so you can login and upload the data, but it would be good to limit this account to only touch data in specific dir and nothing more - no shell, no other locations.
Easiest way is to do it with ssh keys.
0. Add the user on the remote server. Do not set password so it can't login.
1. Generate the key:
$> ssh-keygen -t ed25519 -f rsync_key
2. Copy the .pub key to the server you will be syncing to.
3. Create ~/.ssh/authorized_keys on the remote server with the user that will be holding the data.
in the authorized_keys you must place a line with allowed command + limits and the key like so: