All of the code is being stored in something called a Jupyter Notebook. Jupyter Notebooks are the files with a .ipynb ending, and they work as a markdown file with different code chunks, allowing for a nice combination of text, code, and code outputs. To learn a tiny bit more about Jupyter Notebooks, go to Intro_To_Jupyter_Notebook.ipynb
If you frequently use Python, then the python refresher is probably not for you. If you don't, then the python refresher is designed (by me so don't expect much) to remind someone who knows how to code of python syntax and some basic datastructures that will be useful for graphing. This is by no means an extensive guide on how to use python, it is meant to be a relatively quick reintroduction that will require more research into how to code python if you are starting from scratch.