How do I understand how the following code works?

Go to The Python Tutorial — Python 3.7.9 documentation[^] where you will find all the information you need.

Quote:

How do I understand how the following code works?

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

27.3. pdb — The Python Debugger — Python 3.6.1 documentation[^]
Debugging in Python | Python Conquers The Universe[^]
pdb – Interactive Debugger - Python Module of the Week[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

It works exactly the same way as the last code you showed us: How do I understand the code?[^] and the answer is exactly the same.

Seriously, if you can't work out trivial code like this even with an explanation of the previous version, then you are on the wrong course ... perhaps something more "sports oriented" would fit you better than a "science based" curriculum.