on('load') is not working with jQuery 3?
With Drupal, you don't (need to) use $.on(). It is handled through a combination of Drupal.behaviors, $.once(), and Drupal.attachBehaviors().

First you create a function using $.once() that will add your handler the same as you would with $.on(), but use $.once() to ensure that the handler is only applied a single time:

function addVideoHandler(context) {
  $(context).find(".video").once("add-video-handler").each(function() {
    $(this).load(function() {
      alert("video loaded");
    });
  });
}


Next, this function is called in Drupal.behaviors:

Drupal.behaviors.someArbitraryButUniqueKey = {
  attach: function (context) {
    addVideoHandler()
  }
};


This will be executed on page load, which will add the handler to all .video elements on the page. However, unlike other onload handlers (JS, jQuery), this is also called every time after an Ajax Call is executed, with any new loaded content passed as context. Therefore any newly loaded .video elements will have the handler applied to them - same as with $.on().

Generally, this is all you will need to do. However, if you are loading any content through Ajax in one of your own functions, you will need to pass the newly loaded content through Drupal.attachBehaviors():

$.ajax({
  url: "http://example.com/callback",
  success: function (data) {
    var targetElement = $("some_div");
    // Insert the returned HTML into the DOM, casting it as HTML.
    targetElement.html(data.response);
    // Finally pass the new content through Drupal.attachBehaviors().
    // Note - pass through the HTML, NOT a jQuery object. This can be done
    // by passing the [0]th element of the jQuery object.
    Drupal.attachBehaviors(targetElement[0]);
  }
});


This will call every Drupal.behaviors element, passing it the newly loaded content, which in turn will have the handlers attached. Using $.once() ensures that handlers are not accidentally applied multiple times, for example if someone calls Drupal.attachBehaviors() without passing a value, which in turn will pass the entire document.