The main things are removing the cellular connection and disabling the connection back to the Tesla services. Back in the old days you could pull the SIM card, root the center and driver’s displays, setup firewall rules to block traffic to/from the Tesla servers, and disable the VPN.
This is more difficult with the newer models. You can still pull the SIM, but would need to get creative for root access since it is a continuous game of whack-a-mole between the root methods and patches.
Depends what you want to do. They don’t require a network connection to operate as a vehicle. So if you don’t care about the remote app features (local ones such as lock/unlock still work over BLE), live traffic, streaming music or updates, then a network connection isn’t necessary.
If you do want any of those features, then you would need to either get root access to the gateway and infotainment systems to modify the endpoints or take over the C&C server (formerly named “mothership”) domains and certificates.