I would very strongly advise you to do literally anything other that learn from MMBASIC.
It’s appalling code seemingly written by trial and error, incomprehensible even to experienced developers, the author follows no best practices and accepts no outside contributions.
Furthermore I would have though the actual useful effect you could have on it would be limited.
If you want to learn about development using the Pico native development tools, a much better choice would to take the the PicoCalc Text Starter (C/C++) , and extend one of the examples or create your own Pico-Calc compatible program from scratch.