Confusion regarding jump starting a car

I tried to understand what is going on when one jump starts a car with a dead battery and reading through a lot of explanations, one question remained:

You start by connecting the the + poles with the first (red) cable. After that, you take the second (black) cable and connect it to the – pole of the charged battery.

For safety reasons, you apparently should not connect it to the – pole of the dead battery though, but rather to some bare metal away from the battery. Here is where I am confused. While it is explained everywhere why we do not want to connect it to the battery directly and I think I understood the issue here (risk of explosion), I could not find an explanation for the following:

How does connecting the cable to some arbitrary piece of bare metal charge up the battery? Does that mean the negative pole of the battery is connected to all of the metal in the car, and thus the current flows through the whole car body during the process?

Maybe I also have a fundamental physical misunderstanding here, I hope someone can enlighten me.