Platform/car vs engine

I don't seem to quite understand how a model or car is related to engine.
I can create a car and save it.
I can create engine and save it.
I can create a car and select some saved engine into the trim.
But if I select the engine from trim tab and afterwards edit it do I need to save it again and will it be saved into the engine itself or is the engine stored inside the model without any link into stored engines?
I can create a car without any saved engine. So how is this supposed to be working?
I can create a car and save it.
I can create engine and save it.
I can create a car and select some saved engine into the trim.
But if I select the engine from trim tab and afterwards edit it do I need to save it again and will it be saved into the engine itself or is the engine stored inside the model without any link into stored engines?
I can create a car without any saved engine. So how is this supposed to be working?
