I was wondering how the real systems do start immediately?
A simple Google search reveals, for instance, that Tesla boot time is ways longer than the one of a Raspberry Pi. I'm not even talking about devices other than cars. Most home routers need a minute or two to boot, and where I work, a Xerox machine takes up to ten minutes to boot.
In a case of a car, there are possibilities to mitigate the risk for the driver to find himself in front of a boot screen. The most obvious approach is to avoid shutting down the system in the first place. To save power, the system may be turned off partially, very similarly to what happens when you put your PC on sleep: it takes less than a second to wake it up, but when on sleep, it doesn't consume too much power, although it still receives signals from the keyboard or the mouse or keeps powering the RAM.
Another (complementary) approach is to wake the system up before the person reaches the steering wheel. From the moment the person unlocks the car, it takes about ten seconds to open the door, sit down, close the door and fasten the seat belt—largely enough to boot a Raspberry Pi.
And finally, you have electronics which don't need any operating system and don't have a notion of “boot time.” Arduino is a good example: you can receive information from sensors or display stuff on a screen, while it takes no time to boot.
Finally, with specialized hardware and custom operating system, you can get a boot time of a few milliseconds. DSLRs are a great example of that: unlike cheap point-and-shoot cameras which take seconds to boot, most DSLRs can start shooting photos practically as soon as you turn them on. But they don't use Linux, and they don't use CPUs similar to the one used by Raspberry Pi: instead, it's all proprietary, designed exclusively for a specific task. You won't be able to install an Apache server on your Canon DSLR, and you won't even be able to watch .mpg videos on a Nikon DSLR which cas the capability of shooting (and playing back) videos.