How to calculate spectrum and band boundaries?

Question

Fourier transform

I'm trying to understand how the results of the Fourier transform can be tied to frequencies?
That is, there is a frequency, there is a block that I run through the Fourier, as a result of which I get coefficients (cosine) and these strips jump on my screen.
If you make the conversion back, then everything also plays successfully and the sound comes out almost unchanged (I considered the difference). If you multiply some stripes by something, then the sound becomes muffled or louder.
But how can I calculate which coefficients are responsible for which frequencies? I don't understand how this can be done.