It depends on the implementation. For a client <-> server VPN, it creates an interface on your local machine that corresponds to the network address range for the VPN, and tunnels traffic to the remote end.
For a site to site VPN, two appliances create a tunnel between them, and traffic is routed over that tunnel via the same sort of routing rules you normally use.
> Is the VPN connection setup for the SSH session only?
It can be. It can also be configured for all traffic, or some other combination.
> What if someone needs to have multiple SSH session, going to different networks altogether?
You can have multiple VPN connections to multiple networks. It can get complicated if the VPNs are using overlapping IP space.
> Im thinking it could be pretty impractical to go onto a whole other network to open an SSH session.
I'm not entirely sure why. Millions of people use VPNs every day for a variety of reasons, including SSH. I currently have 8 saved VPN configurations in my wireguard client, and connecting to one is as simple clicking on the client and picking the one I need in the dropdown. Then I SSH as normal, except its to the server's private IP and not public.
It depends on the implementation. For a client <-> server VPN, it creates an interface on your local machine that corresponds to the network address range for the VPN, and tunnels traffic to the remote end.
For a site to site VPN, two appliances create a tunnel between them, and traffic is routed over that tunnel via the same sort of routing rules you normally use.
> Is the VPN connection setup for the SSH session only?
It can be. It can also be configured for all traffic, or some other combination.
> What if someone needs to have multiple SSH session, going to different networks altogether?
You can have multiple VPN connections to multiple networks. It can get complicated if the VPNs are using overlapping IP space.
> Im thinking it could be pretty impractical to go onto a whole other network to open an SSH session.
I'm not entirely sure why. Millions of people use VPNs every day for a variety of reasons, including SSH. I currently have 8 saved VPN configurations in my wireguard client, and connecting to one is as simple clicking on the client and picking the one I need in the dropdown. Then I SSH as normal, except its to the server's private IP and not public.