VPN and networking questions

I have ZeroTier working fine on a Windows desktop PC and Windows laptop. I also haven an Ubuntu Hyper-V VM running OpenVPN on the desktop PC.

What I was thinking about doing was installing ZeroTier on the Ubuntu OpenVPN VM in the hopes of doing 2 things:

  1. Allow devices connected to ZeroTier optionally VPN into the Ubuntu system via the ZeroTier IP in order to get full access to my LAN.
  2. Allow devices connected to ZeroTier optionally VPN into the Ubuntu system via the ZeroTier IP and use it as a traditional VPN.

Here is what I am thinking I need to do, but I was hoping someone could tell me if I am right and then answer some questions.

I would install ZeroTier on the Ubuntu VM and then configure OpenVPN to use the new NIC (presumably eth1). What else do I need to do to the Ubuntu system? Would I leave eth0 configured in OpenVPN, or remove it, or bridge eth0 and eth1 in the OS, or change some route statements, or what?

Assume all the above questions get answered and everything works. While traveling I run ZeroTier on my iPhone and then VPN to the OpenVPN system via the ZeroTier IP. My iPhone can now see my LAN. But the iPhone now has up to three network connections: WiFi, cellular, and ZeroTier (VPN). With WiFi enabled, it takes priority over cellular, so I have two network connections: WiFi and ZeroTier (VPN). How does the iPhone know to route all traffic to ZeroTier (VPN) instead of just using the standard WiFi connection/IP? Is there something I would need to configure

Now take the above situation and assume I am using my Windows laptop with ZeroTier. I once again VPN into the OpenVPN Ubuntu system via the ZeroTier IP address, which means my laptop now has essentially two network connections: WiFi and ZeroTier (VPN). Again, how does the laptop know to use the ZeroTier (VPN) for all traffic instead of the standard WiFi connection/IP?

Thank you for the help!