Learning C++(fstream question)

Hey everyone! I'm going through the book Thinking in C++ which is a great book. I do have a question though about ifstream - ofstream.

#include <string>
#include <fstream>
using namespace std;

int main() {
  ifstream in("Scopy.cpp"); // Open for reading
  ofstream out("Scopy2.cpp"); // Open for writing
  string s;
  while(getline(in, s)) // Discards newline char
    out << s << "\n"; // ... must add it back
}

I understand the while(getline(in, s)) line. If I am correct it reads the line to the \n. But when you copy it to the new file it doesnt add the \n, so you add it manually, correct? But in this code it copies the whole file into a string, and thats where I'm stumped.

#include <string>
#include <iostream>
#include <fstream>
using namespace std;

int main() {
  ifstream in("FillString.cpp");
  string s, line;
  while(getline(in, line))
    s += line + "\n";
  cout << s;
}

The line s += line + "\n"; confuses me. Is it basically saying that s = line including \n? Which allows it to read the whole file? If someone could clear this up for me I would appreciate it. Thanks!