Explanation of the use of async defer for the Google Maps API

I'm trying to understand the loading of the <script> elements in the code snippet at https://developers.google.com/maps/documentation/javascript/tutorial:

<!DOCTYPE html>
<html>
  <head>
    <title>Simple Map</title>
    <meta name="viewport" content="initial-scale=1.0">
    <meta charset="utf-8">
    <style>
      /* Always set the map height explicitly to define the size of the div
       * element that contains the map. */
      #map {
        height: 100%;
      }
      /* Optional: Makes the sample page fill the window. */
      html, body {
        height: 100%;
        margin: 0;
        padding: 0;
      }
    </style>
  </head>
  <body>
    <div id="map"></div>
    <script>
      var map;
      function initMap() {
        map = new google.maps.Map(document.getElementById('map'), {
          center: {lat: -34.397, lng: 150.644},
          zoom: 8
        });
      }
    </script>
    <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap"
    async defer></script>
  </body>
</html>

Presumably, the google.maps.Map object is defined in the latter script. However, that script has the async defer attributes, which according to https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script means that it will be executed asynchronously and after the document has been parsed. But since this comes after the first script, how can the first script instantiate a Map before the definition has been loaded?

Solution

To convert Charlie's comment to an answer, the &callback=initMap at the end of the src of the Google Maps API ensures that the initMap function is called after Google Maps has finished loading.