FFT was such a revelation to me when I was learning about the digital and analogue interaction. The Wikipedia page is really quite about it: https://en.wikipedia.org/wiki/Fourier_series#Fourier_theorem...

I love the visualizations on that page. There were some other cool interactive visualizatiosn on bl.ocks.org, but sadly, that site has be shattered. This is the closest I could find:

* https://observablehq.com/@drio/visualizing-the-fourier-serie...

* https://blocks.roadtolarissa.com/denisemauldin/b0424b73fae8c...