How JNode works and how to create OSes in Java like that?

[Ambijubi]

Maybe this is an offtopic, but I VERY want to know it.
I want to create my own OS like MS-DOS, that brings a little nostalgia, but have some modern features in Java (I know Java and learning C++).
But I can't find any good tutorials how to make operating systems in Java, and probably most accessible operating system in Java that i found is JNode.
This is very good OS, but I didn't find explanations how it works.
Tell me, please, How it works and how to create operating systems in Java?

[tripleo1]

Do some research and document what you find, knowing that much of it will be either wrong or misinformed. You are welcome to ask more specific questions here.

This is very good OS

This is an interesting statement.