[ttt0]

Undertone is something different, but I never explored that concept and what could it be useful for, so I can't really explain it

Not an undertone, it's fundamental frequency. Harmonics/overtones follow a very specific pattern:

https://en.wikipedia.org/wiki/Harmonic_series_(music)

...so in most cases there isn't really any ambiguity on what the actual fundamental frequency is. The best way to demonstrate it is probably with a high-pass filter (aka. low-cut filter). High-pass filter allows only higher frequencies to pass through, or in other words it cuts out lower frequencies. On the example below you can hear that as the lower frequencies are getting removed from the signal, it's still the same note:

https://www.youtube.com/watch?v=50lRE2Bgag0

I wish the filter sweeps were slower, but that's the best example I could find