How is Ctrl + Z interpreted in the Windows terminal?

Question

I ran the program given below. I am aware of the recommendation to use while (std::getline(std::cin, inp)) instead of while (!std::cin.eof()).

#include <iostream>
#include <string>

int main() {

    while (!std::cin.eof()) {

        std::string next_line;
        std::getline(std::cin, next_line);

        if (!std::cin) {
            std::cout << "failbit has been set";
            return 0;
        }

        if (next_line.empty()) {
            std::cout << "input line was empty";
            return 0;
        }

    }

    std::cout << "Success!";

}

Below I give some example input and the program response. ^Z indicates that I typed Ctrl + Z.

In this first example, I typed Ctrl + Z twice across two lines and saw the expected behavior of reaching EOF without flipping the failbit.

Input 1:

this is
my input
and this is the
end^Z
^Z

Output 1:

Success!

In this second example, I was able to add two extra newlines at the end of my input after typing a single Ctrl + Z, however, the eofbit was not toggled.

Input 2:

this is
my input
and this is the
end^Z

Output 2:

input line was empty

In this third example, I saw the expected output of the failbit of std::cin being set. My understanding is that if Ctrl + Z is the EOF, std::getline should set the eofbit of std::cin, but would not yet set the failbit. This is based on line 2 (a) here. The "failbit has been set" output should not be given under normal inputs.

Input 3:

this is
my input
and this is the
end
^Z

Output 3:

failbit has been set

What is Ctrl + Z doing in each of these examples?