Alright, lets see if I can help you out. I'm not quite sure myself on some of the things but I recently just took my first electronics course and I think I might be able to explain some of the things to you.
1) I believe that the car uses dc current under the dashboard. I just figured that since the car battery itself is DC, then all the current going through the car must be DC. At least in all the electrical components.
2) The easiest way to convert 12v to 6v would be to either buy one at Radio Shack, or build a circuit with a series of resistors to bring the voltage down. I'm not sure how to build that circuit but if you type in into any search engine, then they should guide you the rest of the way.
3) A resistor is a circuit element just like a transistor, diode, or capacitor. However a resistor basically disisipates electrical energy meaning that current flows from the positive end to the negative end. This dissipation resists the flow of electic current, thus giving you the ability to raise the voltage in a circuit or bring it down
4) A transistor is a small electronic device that can cause changes in a large electrical output signal by small changes in a small input signal. That is, a weak input signal can be amplified (made stronger) by a transistor. For example, very weak radio signals in the air can be picked up by a wire antenna and processed by transistor amplifiers until they are strong enough to be heard by the human ear. A transistor consists of three layers of silicon or germanium semiconductor material. Impurities are added to each layer to create a specific electrical positive or negative charged behavior. "P" is for a positive charged layer and "N" is for a negative charged layer. Transistors are either NPN or PNP in the configuration of the layers. There is no particular difference here except the polarity of voltages that need to be applied to make the transistor operate. The weak input signal is applied to the center layer called the base and usually referenced to ground which is also connected to the bottom layer called the emitter. The larger output signal is take from the collector also referenced to ground and the emitter. Additional resistors and capacitors are required along with at least one DC power source to complete the transistor amplifier.
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https://www.101science.com/transistor.htm)
5) A diode is a circuit element with two wires or terminals. it allows electrical current to flow in only one direction and is used for converting alternating current to direct current. basically if you have a sine wave signal, a diode will cut the signal in half only displaying the positive wave to be shown. I've learned that the diode is used a lot in radios because it takes the radio signal and cuts it in half so that the radio itself can identify the carrier frequency. Basically Diodes are an electrical device which are used to isolate current flow by controlling (blocking or passing) its flow.
6) A capacitor is a circuit element that stores electrical energy, kind of like a battery (I can explain it all to you but I think this website does a better job)
https://electronics.howstuffworks.com/capacitor.htm
7) I'm sorry but I have no idea what a low voltage amplifire does. I personally have not even heard of one yet.
Anyway I hope this helps a little and if you have any more questions. Please do not hesitate to ask. Enjoy and good luck with what you plan to do with this knowledge.