[WxPython] Puzzled by classes

[Barrowman]

I have been learning to create GUI's using tkinter for some time and have created around 12 which I am happy with.
I have not used classes but see more and more examples of code using them.
I had not seen anything to persuade me to use classes or to change from using tkinter to using wxPython.
I found wxPython to be difficult for me to work with initially which is why I chose tkinter.
Recently I saw that wxPython seems to me very versatile so started to look here

http://zetcode.com/wxpython/menustoolbars/

Looking at these examples I cannot figure out how anything is instantiated.
The class is defined but seems to be used directly but I don't understand this:

def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    

Could someone explain this please?
Thanks