I've concluded the AJAX basic course but can't identify the similarity with the following pattern. Could you help?

<script>
    var xhr = new XMLHttpRequest(); 
    xhr.onreadystatechange = function () {
             if (xhr.readyState === 4) {
                 document.getElementById(‘ajax’).innerHTML = xhr.responseText; 
            }
        }; 
         xhr.open(‘GET’, ‘sidebar.html’);

            function sendAJAX() {
             xhr.send();
             document.getElementById(‘load’).style.display = “none”;
            }
</script>

<script>
   $.ajax({
      type: "POST",
      contentType: "application/json",
      url: "/api/search",
      data: JSON.stringify(search),
      dataType: 'json',
      cache: false,
      success: function (data) {
         //some code
      }
   }); 
</script>