Help understanding AEM JavaScript Ejst and CQ

I am trying to start writing my own custom xtypes in AEM.

In order to do so I have realized that I need to extend my JavaScript programming skills and specifically understand CQ JavaScript custom classes and methods.

I am not an advanced JavaScript programmer.

In the following development article https://helpx.adobe.com/experience-manager/using/creating-aem-multifield-components.html
 there is some JavaScript which I am trying to understand and that I will eventually have to customize.

For example in CustomWidget.js

Ejst.CustomWidget = CQ.Ext.extend(CQ.form.CompositeField, {

How would I find documentation to help me understand what Ejst is.

And off course its methods.

Then there is the CQ JavaScript. Where the expression starts with CQ.EXT and so on.

For example on these forums I found the following JavaScript to create a url and call it.

    var url = CQ.HTTP.noCaching("/content/whatever/all."+query+".search.html");
    CQ.HTTP.get(url);

The CQ.HTTP.get works, but how would I find further documentation or help to using CQ specific JavaScript.

Best regards

Clive Stewart