How can I determine what header calls another header in c++?

I am reading from here : Can std::string be used without #include <string>?, <iostream> is calling <string>. But I do not see any includes of <string> in Standard library header <iostream> from c++ standard: https://en.cppreference.com/w/cpp/header/iostream. In <iostream> is only included <ios>, <streambuf>, <istream> and <ostream> according to that documentation. So this code works (file foo.cpp):

#include <iostream>int main(){    std::cout << "enter srting\n";    std::string out; //std::string should not be included according to standard documentation    std::cin >> out;    std::cout << "test, " << out;}

But I have to try to see dependecies generated from gcc:

cc -H foo.cpp |& vim -

and the output is (I have regex out files with string in it):

. /usr/include/c++/8/iostream..... /usr/include/c++/8/bits/stringfwd.h...... /usr/include/c++/8/string....... /usr/include/c++/8/bits/basic_string.h........ /usr/include/c++/8/ext/string_conversions.h....... /usr/include/c++/8/bits/basic_string.tcc

So I can see that in the end, many "string" headers are indeed included

1. (so should I trust that documentation, when the did not mentioned <string> header in "Includes" list of <iostream>?).

Some of them are deeper on the #include stack (term from gcc manual), which brings me to question,

2. what calls what? And what is the "true" header, that define std::string of them? (is it /usr/include/c++/8/bits/basic_string.h?...)

from this question Why does omission of "#include <string>" only sometimes cause compilation failures?, they mentioned:

Some compilers on some platforms may on some time of the month compile even though you failed to include the header

But from the upper output of the "string headers", there is multiple of them , so how's possible for a compiler to compile only sometimes? Which of these headers are really important for successful compilation?

3. How to orient in cpp headers, which are meaningful for compiler, and could be tracked their "#include stack" (i.e. other meaningful headers)?

EDIT:If it depends on my specific implementation of my stdlib++, then I want to know how can I determine from source whether that inclusion is made before I try to compile. Not by "If it compiles, then it works".