>https://www.musictheory.net/lessons

Nice website! I will have a look. Unfortunately, their mobile version is for iOS only ;-(

>why your flute's FFT visualization has multiple bumps

I am not surprised it has multiple bumps, I am surprised that its dominant frequency is not at the expected note. I suspect that it's just because I am holding my fingers in a wrong way, but I don't know how to tell.

>get a copy of Curtis Roads' Computer Music Tutorial

Thank you!