How do people write operating systems?

Question

Operating Systems

How do people write their OS? There are those who write at home alone.
Suppose I take ideas for design from Tanenbaum's book, look at the Minix code, and then what?
Will it be enough to read the documentation for the processor (for example, Intel 4000 pages)?
Does the computer vendor provide any examples of system software? Or how do people start writing their own OS?

Please, jokers and js-macaques do not answer.

Answer 1

Take and write. They read books on theory, take ready-made sources, try to isolate the pieces in them that were written about in books and trace their interaction, draw up a project of their axis in their mind, and do development by components.
Long, tedious, difficult... and completely pointless work, except just for fun.

Answer 2

There are those who write at home alone.

There is. These are God's chosen people (or just schizophrenics). And they write OS to communicate with him.
Link to habr - TempleOS: the biblical operating system that ...

Answer 3

The full processor documentation should be read if you are writing a compiler.
And for the operating system, you need to read not so much the processor as how to work with devices, at least the screen, keyboard, disk subsystem, and the actual BIOS / UEFI functions

Answer 4

It takes quite a long time to write a full-fledged OS and it is impractical in our time. Read what the Oberon 2013 project is, in which the author adapted the OS with a graphical user interface to run on an FPGA (FPGA).