How I figured out where play() and stop() are defined

currentSong.play();

hereComesTheSun.play(), 

new Song("Here Comes the Sun", "The Beatles", "2:54").play()  

Song.prototype.play()

//Song constructor. Used to create objects that have a title, artist, duration, and isPlaying status.

function Song(title, artist, duration) {
  this.title = title;
  this.artist = artist;
  this.duration = duration;
  this.isPlaying = false;
}

//Shared Song constructor prototypes. play() changes isPlaying to "true". stop() changes isPlaying to "false". toHTML() adds a list item to the ordered list in the index.html file. This list item is given the class "playing" if the play() function has been called. 

Song.prototype.play = function() {
  this.isPlaying = true;
};

Song.prototype.stop = function() {
  this.isPlaying = false;
};

Song.prototype.toHTML = function() {
  var htmlString = '<li';
  if(this.isPlaying) {
    htmlString += ' class="playing"';
  }
  htmlString += '>';
  htmlString += this.title;
  htmlString += ' - '
  htmlString += this.artist;
  htmlString += '<span class="duration">'
  htmlString += this.duration;
  htmlString += '</span></li>';
  return htmlString;
};

//Playlist constructor stores an array of songs and an index number to access those songs.

function Playlist() {
  this.songs = [];
  this.nowPlayingIndex = 0;
}

// Shared Playlist constructor prototypes. 

//add() uses the push method to add a song to a Playlist instance, playlist (defined below). add() takes a parameter, (song). That comes from an instance of the Song constructor, which is created below. So, when you call the add(), you do this: add(new Song("Here Comes the Sun", "The Beatles", "2:54")). 
//The resulting array item looks like this:
// [ 
//    {
//    title: "Here Comes the Sun",
//    artist: "The Beatles",
//    duration: "2:54",
//    isPlaying: false
//  }
// ]

Playlist.prototype.add = function(song) {
  this.songs.push(song);
};

//play() first selects a song from the playlist array and stores this song in the variable currentSong. Remember that the song in the playlist is an instance of the Song constructor. So currentSong.play() is equivalent to hereComesTheSun.play(), which is equivalent to new Song("Here Comes the Sun", "The Beatles", "2:54").play()   So you're calling an instance of Song.prototype.play(). As you can see above, Song.prototype.play() sets isplaying to "true".

Playlist.prototype.play = function() {
  var currentSong = this.songs[this.nowPlayingIndex];
  currentSong.play();
};

//just as currentSong.play() is equivalent to Song.prototype.play(), currentSong.stop() is equivalent to Song.prototype.stop() which sets isPlaying to false.

Playlist.prototype.stop = function(){
  var currentSong = this.songs[this.nowPlayingIndex];
  currentSong.stop();
};

Playlist.prototype.next = function() {
  this.stop();
  this.nowPlayingIndex++;
  if(this.nowPlayingIndex === this.songs.length) {
    this.nowPlayingIndex = 0;
  }
  this.play();
};

Playlist.prototype.renderInElement = function(list) {
  list.innerHTML = "";
  for(var i = 0; i < this.songs.length; i++) {
    list.innerHTML += this.songs[i].toHTML();
  }
};


var playlist = new Playlist();

var hereComesTheSun = new Song("Here Comes the Sun", "The Beatles", "2:54");
var walkingOnSunshine = new Song("Walking on Sunshine", "Katrina and the Waves", "3:43");

playlist.add(hereComesTheSun);
playlist.add(walkingOnSunshine);

var playlistElement = document.getElementById("playlist");

playlist.renderInElement(playlistElement);

var playButton = document.getElementById("play");
playButton.onclick = function() {
  playlist.play();
  playlist.renderInElement(playlistElement);
}