How is this function create ? (shuffle part)

/* INSTRUCTIONS
To run this file, click in the Console below and type: node 3_deck.js 
If the console isn't visible, click the View menu above and choose Show Console.

You can clear the console by typing `clear` and pressing enter.

If your program is stuck in an infinite loop, you can break out of the program by typing ctrl + C.
*/

// Note: We've supplied you a basic function that takes an array of elements and shuffles their order in the array. It's a really useful, but somewhat complex algorithm -- look in the teacher's notes for links to an explanation for how it works. You'll use this function to shuffle a deck of cards that you'll create.
function shuffle(arr) {
  var j, x, i;
  for (i = arr.length - 1; i > 0; i--) {
      j = Math.floor(Math.random() * (i + 1));
      x = arr[i];
      arr[i] = arr[j];
      arr[j] = x;
  }
  return arr;
}

/* Complete the function createDeck(), which we've started for you below. The goal is to create a multi-dimensional array of 52 elements (52 cards), and return them shuffled when the function is called. To simplify creating the deck use two for loops -- one inside the other -- to build a multidimensional array.
*/
// 1. Add a for loop -- to loop through each element in the suites array
// 2. INSIDE that loop, add another loop that loops through elements in the ranks array.
// 3. Inside the second, nested loop, create a new array named card, which is composed of a rank and a suite. For example ['King', '♥︎'].
// 4. Push that card onto the deck array. Once both loops complete running, the deck array will hold 52 elements, and each of those elements are themselves an array.
// 5. Finally, pass the new deck to the shuffle() function, and return the results.
function createDeck() {
  var suites = ['♠︎','♣︎','♥︎','♦︎'];
  var ranks = ['Ace','King','Queen','Jack','10','9','8','7','6','5','4', '3','2'];
  var deck = [];
  // add your code below here:
  for (let i=0; i<suites.length; i++) {
   for (let j=0; j<ranks.length; j++) {
     let card = [];
     card.push(ranks[j], suites[i]);
     deck.push(card);
   }
  }
  return shuffle(deck);
}
// 6. Call the createDeck() function and store the results in a new variable named myDeck
let myDeck = createDeck();

/* 7. Use a for loop to loop through the deck and list each card in the order the appear in the newly shuffled array. Use the log() method to print out a message like this, once for each card:
"7 of ♥.︎"
*/
for (let i = 0; i<myDeck.length; i++) {
  console.log(myDeck[i][0]+ ' of ' + myDeck[i][1]);  
}

 var j, x, i;
  for (i = arr.length - 1; i > 0; i--) {
      j = Math.floor(Math.random() * (i + 1));
      x = arr[i];
      arr[i] = arr[j];
      arr[j] = x;
  }
  return arr;
}