Well... there is that one big integrated circuit in a metal can which does all the complicated stuff.
When I had computer 101, we learned that a computer had a CPU, short-term memory (RAM), long-term memory (hard/floppy drive) and can start & stop programs at will until you turned it off. Micrprocessors in cars are similar because they have a CPU and memory (RAM and flash memory) ( think of the Arduino things) and only operate one kind or system or devices in a vehicle (engine, transmission, electrical (lights/security/heat/windows) entertainment, etc. Each begins running a single, but very sophisticated, program when you turn on the main power, and runs it until the power is turned off.