I just use the primary interface for diagnostic.
My main problem is how to start my tunnel before any user login on the UI (assuming FileVault is disabled). The use case is to be able to remotely, securely, and reliably access the Mac through the tunnel. You could call this feature "always-on", "server mode", or "unattended mode".
To test if this feature works, I try to connect with SSH on the VPN IP. I cannot connect, so for purpose of experimentation, I try to connect with SSH on the primary interface IP, where I find out with scutil --nc list that my tunnel is disconnected, and that I can start it with scutil --nc start and then connect with SSH on the VPN IP. Without logging in on the UI.
So, I, as a developer, used the primary interface to find out that in theory the system might be able to automatically start the tunnel on boot before user login, but for the end use-case this primary interface may not be reachable.
So now I wonder, why can't the tunnel start itself on boot? Am I using it wrong? Is it a missing feature, a technical limitation, or a restriction that exists on purpose? I am especially confused because in the posts I linked in my first message, it is said that "a Network System Extension on macOS is started when the system starts", and that "network extension [will] run before user session".