What happens if the main battery gets connected backwards?

2008 Mitsubishi Nativa (Pajero?) now no power coming from alternator after my experienced mechanic hooked up the car’s battery backwards.

He saw the spark he said but didn’t register as he was distracted, and ran the motor.

What has been damaged? Blown fuse(s)? Other damages?

I installed a known-good new battery, and connected it correctly.

The car will run but if battery wire is removed while running, then motor dies. I did this to check the alternator. I am trying to resolve this issue myself.