How does a soft reset of a smartphone with Android really work?

Does it only trigger some kind of "shutdown -h now"? Or are there any hardware mechanisms involved? I want to know, what happens, if I shutdown the smartphone such as you can do with holding the "start" button for 10 to 20 seconds in most cases. More concrete: What happens, if I press the power button on my Huawei P20 for 10 seconds? What steps are taken exactly?