wiring fundamentals further considerations

On this topic i wanted to understand what is said between the first 32 seconds and the rest of the video, what is mentioned here is about the ECU having a hold power installed when constructing the harness, and the rest of it is saying when you turn off your ignition basically you cut power to the ECU and the Main relays when the ignition it switched off. Is this power hold supply that keeps the ECU alive when power is turned off to allow it to remove power and shut down done by installing capacitors somewhere in the circuit or how is it done?

The idea is the ECU may want to remain powered after the ignition is switched off. Perhaps it controls fans or pumps that want to run after the engine stops to avoid heat soak. When the after-run is complete, the ECU will basically turn itself off by releasing the power hold.

It doesn't need capacitors, it's still powered internally.

I would expect it to be programmed to poll (ask) each function if it is still requiring/using power, if so it will stay 'switched on' until no response requires power, when it will turn itself off. Same principle as your computer, when you turn it off, it doesn't shut everything down at once, but in a process that allows each device to close safely before the operating system actually turns off.