Executing a single command:
ssh USER@HOST ls
sudo requires interactive shell, it can be enabled with -t parameter:
ssh -t USER@HOST sudo ls /root
[sudo] password for USER:
https://zaiste.net/posts/a_few_ways_to_execute_commands_remotely_using_ssh/
On a system without an ssh-agent remember the ssh key password and start an ssh-agent
eval `ssh-agent` && ssh-add ~/.ssh/id_rsa
scp between 2 hosts: https://superuser.com/questions/686394/scp-between-two-remote-hosts-from-my-third-pc