How can I provide dynamic values for an 'extend'? (or 'how does extend work in this case' ?)

I'm looking at a class (this one). The doco for it suggests you can extend the class to change a property of the class. That is true, it works for me.

However what I want to do is dynamically provide the values for the setting.

I've got two questions.

First

How can I adapt the demo so that I can dynamically supply the values used.

Second

I realise when I look at the class, this seems to be the significant part of the code ...

  ajax (url, data = {}, method = this.method) {
    const ajaxSettings = assign(
      {},
      {
        contentType: false,
        processData: false,
        xhr: () => {
          const xhr = $.ajaxSettings.xhr();
          xhr.upload.onprogress = (event) => {
            this.didProgress(event);
          };
          this.one('isAborting', () => xhr.abort());
          return xhr;
        },
        url,
        data,
        method
      },
      get(this, 'ajaxSettings')
    );

    return this.ajaxPromise(ajaxSettings);
  },

... I'm not confident that I understand how the 'extend' is doing what it's doing . ajaxSettings is used within the ajax function so how does the extend reach within the function and merge the value provided in the extend into the hardcoded values in the function ?

Hope the question makes sense ... I'll happily settle for an answer to 'First' if you can't manage 'Second' without writing a book ;-)