I will always have a soft spot in my heart for Django, the python web framework, even though I don’t use it anymore. https://www.djangoproject.com/
When I was still learning to code, I spent hours and hours and hours poking around the Django source code. In particular I was fascinated by the metaprogramming used in the Model and Query objects, and I ended up learning a ton about how Python works under the hood.
https://github.com/django/django Django's codebase is a pleasure to read. The directory structure is intuitive to navigate, and test coverage is great.