As I understood, 12-V car system refers to automotive system that runs with a 12-V battery. What are the basic differences between a 12-V and a 24-V (actuators, ECU...) ?
24V is used on trucks as the starter needs enough power for the big diesels : approximately 1000A. If that was at 12V then it's double the current which means thicker cables.
The disadvantage was that the bulbs needed longer filaments for 24V and the vibration (road and engine) caused them to fail often, this was helped by supported filament bulbs so that they lasted longer, now LEDs have helped even more.
In the automotive industry every single cent saved could be worth millions to a company because of the large volumes. Higher voltages saves copper by reducing the cable area. But for a small car 12V is usually the standard. Trucks usually use 24V. Working for a company that makes ECUs* we make the main regulator cover the whole range if it's used for both cars and trucks. Sensor inputs/outputs, voltage protection etc, are usually handled by mount options on the PCB.
* Electronic Control Module NOT Engine Control Module