This post explains what a Python module is and how to use it to drastically extend the capabiltiies of your code. Plus: how to make your own! How to import, create, or use a Python module ...

I've narrowed down that mitsuba_ext is "mitsuba_ext.cp310-win_amd64.pyd". Explicitly importing that specific file works without any problem using blender's python exe, but inside the blender terminal, ...