Notebooks to python

Hi,

may I ask a beginner’s question regarding the fastai library?
I had a look at the fastai repository on GithHub and the package on PyPI.
What I saw is that fastai is developed in notebooks, which are on GitHub in the nbs folder. However the fastai PyPI package has python files. I assume the python files are generated from the notebooks. How is this done? Is there a tool for this?

thanks
Norbert