I'm going to be this guy, but isn't it just Pythagoras theorem with a slight twist which is taught at 11 - 14 year old levels?
It only sounds complicated because of the words used like "complex", "imaginary", "real".
So if you studied Pythagoras at school and someone (a YouTube video) says you just have to do Pythagoras on the i multiplier and the other number, it would be fairly easy if you understand Pythagoras?