Module std

A.1

module std is not in C++20; it may go to C++23.

A.2

Some compiler does implement the module std. To use it, it would be something like

export module std;
export import std.regex;  // <regex>
export import std.filesystem; 
export import std.memory;
export import std.threading;

export import std.core;  // all the rest

A.3

make your own std header, and then #include "std.h"

//std.h
#include<iostream>
#include<string>
#include<vector>
#include<list>
//...any more.