I am currently trying to access the source code for PyQt5's QTabBar Class but am not entirely sure how to do so. I've attempted using the inspect module and have examined the PyQt5-5.15.4.tar.gz file found on pypi.org, but I still haven't found the file where the QTabBar is defined in Python. I understand that the C++ source code is readily available.
There is no such code. PyQt5 is a Qt wrapper, this is through SIP that creates CPython code (python implementation written in C) that wraps each class. The same happens with PySide2 that Shiboken uses.
In general, it is not necessary to know how the Qt code implements, but only to read the docs (The docs are very good) that indicates the behavior of the code. Much of Qt's public API code is simple and straightforward to understand, but the private API where the magic happens can be changeable.