I am a beginner, and I am wondering about how to power different circuits with one supply.
Let's say we have a car battery with like 14v, how does that volatge get spread across the various radio, screen, and audio circuits. I'm assuming they don't all run off 14v.
This tends to happen with modern computer boards.
For an analogy, think about the mains supply in your house. Most items need a lower voltage, so you have various "wall wart" or "plug pack" adapters like cellphone chargers that you plug into the mains and get a low voltage out the other side. All of these adapters are various kinds of voltage step-down converter.
Similarly, a modern computer board may have a 12 V supply coming in and have it routed around the board like a 12 V "mains". Various chips that need lower voltages, like 5 V, or 3.3 V, or 2.5 V have local voltage step-down converters that take the 12 V "mains" coming in, and produce the required lower voltage going out.
The mode of operation of mains adapters and local voltage step down converters on a computer board is very similar. They all tend to be a kind of "transformer" with special circuitry to control and regulate them.