You need electrons to flow in one side and out the other. Batteries do not hold electrons – they move them. A battery always holds the same number of electrons.
The car battery moves electrons in one direction, to charge it you must move them in the other direction. You need to connect to *both* terminals either way.
When you connect negative to “ground”, you connect it to the car’s frame, which is connected to the other battery’s negative terminal. You *are* connecting to the negative terminal, just indirectly.
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