Ah OK, the fundamental frequency, not EQ, it makes more sense now
Well, reading your comment I think that you got the principle really, yes most of the time it's the loudest frequency which is matching the fundamental note that compose the sound.
This is based on the relation between musical theory notes and they frequency equivalences within the frequency spectrum, each notes is matching a specific frequency like for example the A5 reference tone note A (440 ) is matching a frequency of 440 Hz.
Now when it comes to a single note played back by a traditional instrument it's rather easy to see it on a FFT spectrum analyzer, but with more complex sounds like chords or bass & elaborated synth sounds, it's a bit less obvious.
We can compare this to playing a single note and a chord in music theory : in a chord there's a root note that is the fundamental and other notes that are related to this fundamental and playing harmonies, the principal is the same when looking at FFT Spectrum and frequencies, we can see fundamental frequencies but also the harmonics frequencies that a sound generates.
So if you play one A5 note, you will see a louder, dominant 440 Hz frequency in your FFT analyzer but also all the other frequencies : the harmonics that this note is generating.
NB : All of this is quite well explain in the Level 2 of Understanding EQ with Protoculture BTW, I suggest you'll have a watch to it ( link in my previous post )
One very nice and free FFT Analyzer that can display notes + frequencies information is SPAN from VOXENGO, you can grab it here https://www.voxengo.com/product/span/
See also the chart below as a reference for notes to frequencies but you can find many sheets & other information like this on the web
ChasinConversionChart.pdf (26.9 KB)