Understanding how pmusic works

I have been researching the use of pmusic as an alternative to produce high accuracy frequency spectrums of signal waveforms and its possible use as an alternative to FFTs which can have limiting bin widths.

My focus is on how accurately it produces harmonics on the x-axis. For example I am measuring a 50Hz mains signal and I want to know how accurately the signal actually sits bang on 50Hz and see if there is any variance by a few Hz. I know that pmusic ruins the amplitude information (not sure how or why yet, please explain if possible).

I also see that the harmonic peaks become much sharper when more harmonics is used with pmusic. When looking only at the fundamental, it is not a very sharp peak and actually moves around quite a lot (sometimes 40 to 60Hz over my 20 waveform samples). Compared to using pmusic on 2, 5 or more harmonics which are much sharper peaks.

I have come across something interesting that I don’t understand and this is my main question. Because I am using data consisting of 20 waveforms captured over a minute from mains, my [wf_YData] variable is 150000x20. I have many loops in my code for all of the tests I am doing and methods I’m comparing and when I pass all 20 waveforms to pmusic by giving it [wf_YData], the resulting harmonic peak is very sharp. Whereas when passing only one waveform (150000x1) to pmusic which produces a much fatter peak.

Therefore,

Why does passing pmusic more waveforms, produces much sharper harmonic peaks?

Why does pmusic also produce sharper harmonic peaks when it is told to analyse more number of harmonics? (analysing the first 20 is better than just the fundamental on it own)

Please Briefly explain of link a good source: how pmusic and Pisarenko method works, mainly why it produces much sharper harmonic measurements in the frequency axis, but ruins/loses most of its x-axis amplitude information?

^ pmusic is given 1 waveform and told to look for 1 harmonic (fundamental)

^ pmusic is given 1 waveform and told to look for the first 3 harmonics (only first shown)

^ pmusic is given 1 waveform and told to look for the first 3 harmonics (all 3 shown)

^ pmusic is given 20 waveform and told to look for 1 harmonic (fundamental)

Thanks