What are the components of a panel applet in Mate?

Question

This is my first post on Stack Overflow - I'm not a professional developer, and when I have a problem in one of my little experiments it's always rather simple and I normally find the solution among the answered question. This time my search hasn't been successful, so I'm afraid I'll have to make my ignorance publicly known...

I am interested in learning to develop Python applets for mate-panel. I have found a number of examples around the web, including here on SO. However, none of the examples I found is accompanied by enough commentary for me to make sense of what the code does. Moreover, I have tried to run some of them, but I can't get them to work, and it's hard to troubleshoot not knowing the mechanics. Here is what I found so far:

• This question links some documentation that sounded promising, however the link appears to be dead now.
• A github repository called MATE University promises to be targeted at programmers of all abilities - unfortunately it only contains some minimal, uncommented code. The link to the relevant "documentation" is also dead. Overall, it looks like an abandoned project (my impression is confirmed by the fact that it's in a legacy archive).
• The MATE wiki would seem like the place to go, but again there is only an uncommented example.

I found also a number of forum questions by people troubleshooting their code, but none of them took me very far.

Ideally, I would appreciate it if somebody could point me to a helpful reference. Alternatively, I would be super-grateful if somebody could answer my questions on the example in the wiki.

For reference, here is the code for the applet, in Python:

#!/usr/bin/env python

    # this code is based on example appet by Vincent Untz for GNOME Panel 3
    # https://git.gnome.org/browse/gnome-panel/commit/?id=5ad4d9e

    # ensure we are using Gtk 2, not Gtk3
    # this will print a warning but everything should work
    import gi
    gi.require_version("Gtk", "2.0")

    from gi.repository import Gtk
    from gi.repository import MatePanelApplet

    def applet_fill(applet):

        # you can use this path with gio/gsettings
        settings_path = applet.get_preferences_path()

        label = Gtk.Label("My MATE applet in Python")
        applet.add(label)
        applet.show_all()

    def applet_factory(applet, iid, data):
        if iid != "TestApplet":
           return False

        applet_fill(applet)

        return True

    MatePanelApplet.Applet.factory_main("TestAppletFactory", True,
                                        MatePanelApplet.Applet.__gtype__,
                                        applet_factory, None)

My questions on this are:

1. All the examples I have seen contain at least two functions called "applet_fill" and "applet_factory". It's easy to see that "applet_fill" takes an applet object that behaves like a Gtk container, and that here goes the code that fills the applet with widgets. Is that all or is there more to it?
2. What's the role of "applet_factory"? What does the return value mean? What is the "iid" argument? Is "data" a dictionary? I suspect "data" might be the last argument of the "factory_main" function below, is that correct?
3. Speaking of which: what is "factory_main" and what are its argument? I imagine it is the function that executes the applet, is that correct? I tried calling help() on it, the output was depressingly unhelpful.

Beside the python script, the wiki also 'introduces' two additional files:

    [Applet Factory]
    Id=TestAppletFactory
    InProcess=false
    Location=/home/user/applet/testapplet.py
    Name=Test Applet Factory
    Description=A MATE Python Applet example factory

    [TestApplet]
    Name=Test
    Description=A MATE Python Applet example
    Icon=mate
    MateComponentId=OAFIID:MATE_TestApplet;

and:

[D-BUS Service]
Name=org.mate.panel.applet.TestAppletFactory
Exec=/home/user/applet/testapplet.py

My question here is embarrassingly basic: what is the function of these files and what goes in them?

I hope the question wasn't too sub-standard. Any help will be very much appreciated.