How to download a file from server using SSH Linux?

scp /path/to/local/file username@hostname:/path/to/remote/file

Copy something from some system to some other system:

scp username1@hostname1:/path/to/file username2@hostname2:/path/to/other/file

Copy something from another system to this system:
scp username@hostname:/path/to/remote/file /path/to/local/file