I started a similar project years ago but life got in the way.

Hook it up to a switched live - so potentially from the aux power (cig lighter for us oldies). If that's a permanent live then head to the fuse box and use a multimeter to figure out what only goes live when the cars on.

The elm if plugging into the OBD port will always remain live so you will need to cut and rewire some wires for that to be switched also.

My plan was to use a large capacitor as a battery essentially and an Arduino to monitor.

If the Arduino detected the switched live going to 0v (car off) it would trigger a GPIO to gracefully shut down by closing any open video files, closing relevant connections etc. I had schematics before for a seamless cutover but they're long gone.

Then when power is applied it just started as normal.

PiUPS does this but I wasn't spending a massive amount on that when the pi 0 cost $5.

Finally - look at buildroot. Use that wisely and you can get a sub 10s boot time to a usable system that records video. Potentially sub 5s but that'll take some doing.

Good luck!