How to implement I/O from scratch?

Question

C++ / C#

Without any libraries and headers.
Who can tell how this is possible?
(After all, the low-level input libraries were somehow written).

Answer 1

After all, the low-level input libraries were somehow written

Bingo! It remains to find and read their code. glibc has it open, for example.

Answer 2

In what sense, without libraries? What level of access? It can be at the controller level, at the kernel level, at the user level, at the scripting language level, etc. Everywhere has its own nuances.
> Without any libraries and headers.
a very useless exercise, only if you do not write your own driver