How the operating systems are made?

Answer 1

https://m.wikihow.com/Make-a-Computer-Operating-System

Answer 2

Ur question made me read this Quora post, this guy has also shared his simple x86 64 bit OS in bitbucket,
(disclaimer:not that simple though)

here's the link, https://www.quora.com/How-can-I-create-an-operating-system-1

but note that he had a bachelors degree in computer science , and he made this in his master's , so meaning that you need to have ur concepts figured out properly before starting something big like this,