Z Axis Height Calibration

[Differential_faw]

So I am sooo confused. There seems to be mutliple ndifferent processes to fix things on a mechanical and a software level. How do you tell which problem.younhabe if any at all and which process to use. I also don't understand any of the processes. People keep saying home is not 0 but z is 0 or vice versa. Then folks are saying need to hit the level plate button then move up a couple mm??? This would seem to leave plate above fep as far as I am mis understanding. Can someone please help or am I too beyond lost to help?