XMLHttpRequest object event handlers

Having just finished AJAX basics and moving into this course I got very confused about the function written in this code to create the XMLHttpRequest object. It took me a while to realize what was so confusing, but I figured out that there is a method called on the XMLHttpObject that is not used in the AJAX basics course, xhr.onload. It seems this method is actually an event handler that has been recently added by various browsers? Furthermore, when I went back to the AJAX course I recalled we were told there were 4 steps to creating an AJAX request - 1. create the object, 2. write the callback, 3. open the request, 4. send the request. This code does not create the callback until after opening the request, yet another confusing point. I think it would be helpful if at a minimum a link is included in the teacher's notes to some content on this new method, .onload - as otherwise it is confusing. I found some info on mdm here - https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequestEventTarget/onload - however it was not particularly informative. Perhaps you have access to something better?