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.

This is handled by the inverter and charging modules, some use FPGA chips others use dedicated ASICs, but it doesn’t require anything wild in terms of raw compute power, mostly up to having good algorithms to handle the situations correctly. Nothing more than a modern ICE engine which needs to very precisely manage intake and exhaust cam phasing, ignition timing, intake pressure, and multiple injections per cylinder/cycle along with monitoring a multitude of sensors to keep everything in tolerance. In terms of simplicity, the first automobiles at the turn of the century were electric before the ICE caught on thanks to the advent of the electric starter and limitations in battery technology at the time.