VNC allows to take control of a remote desktop... but this desktop should be
in the local network because VNC is not secured. SSH is secured but does not
let you take control of the a running session (a new session is created).
In all case, you need to know the IP address of the computer you want to
connect to. DuckDuckGo (for instance) can easily tell it:
https://duckduckgo.com/?q=ip
If the computer is behind a NAT router, you need to configure the router so
that:
1) it always attributes the same IP address to this computer;
2) the port you use (by default: 22 for SSH, 5900 for VNC) is redirected to
this IP address.
