So, let's say my staff uses a car spare-part product on an operational car. How to track those spare parts?

You can create a new location called Inventory Loss or Internal Consumption. Then you can configure the location type as Inventory Loss.
Now whenever you are using the spare parts for the operations cars, create an Internal transfer and move the stock there.

I see... before this, I already configured my Internal Consumption Location type as Customer Location. So it must be Inventory Loss and not Customer Location? Or both can work too? @Balagopal_R