Issues with standard input in C

I'm making a simple program in C that reads an input. It then displays the number of characters used.

What I tried first:

#include <stdio.h>

int main(int argc, char** argv) {
    int currentChar;
    int charCount = 0;

    while((currentChar = getchar()) != EOF) {
        charCount++;
    }

    printf("Display char count? [y/n]");
    int response = getchar();

    if(response == 'y' || response == 'Y')
        printf("Count: %d\n",charCount);
}

What happened:

I would enter some lines and end it with ^D (I'm on Mac). The program would not wait at int response = getchar();. I found online that this is because there is still content left in the input stream.

My first question is what content would that be? I don't enter anything after pressing ^D to input EOF and when I tried to print anything left in the stream, it would print a ?.

What I tried next:

Assuming there were characters left in the input stream, I made a function to clear the input buffer:

void clearInputBuffer() {
    while(getchar() != '\n') {};
}

I called the function right after the while loop:

while((currentChar = getchar()) != EOF) {
    charCount++;
}
clearInputBuffer();

Now I would assume if there is anything left after pressing ^D, it would be cleared up to the next \n.

But instead, I can't stop the input request. When I press ^D, rather than sending EOF to currentChar, a ^D is shown on the terminal.

I know there is a probably a solution to this online, but since I'm not sure what exactly my problem is, I don't really know what to look for.

Why is this happening? Can someone also explain exactly what is going on behind the scenes of this program and the Terminal?