You'd also think that if the AirPods adjust by different frequencies, they would give users the ability to tweak the settings by frequency!
Anyone yet found an Apple declaration of frequency adjustments or user-controlled adjustments by frequency?!
The sound is adjusted by frequency based on the audiogram generated by the hearing test or manually entering information from a professional hearing test. AirPods have already been doing that even before the latest release via the "Headphone Accommodations" function. What's new is the 'Hearing Aid" functionality that actually works reasonably well.
My hearing loss fits the pattern of Sensorineural hearing loss. That is, my hearing is relatively good at lower frequencies but rapidly trails off at higher frequencies. The result is that it is difficult for me to make out words because the consonant sounds mostly go away. I can tell that people are talking, just not the words they are saying.
At the low end of the audiogram spectrum I have a loss of about 20db. But at the higher-end (2-4KHz) my loss is about 85dB. Imagine listening to music with the bass slider cranked all the way up and the treble slider cranked all the way down. That's my life. My APP2 can't make up for all that high-frequency loss in my case. But it does a reasonable job. My real prescription hearing aids do much better.
So, yes, the APP2 are adjusting for different gains at different frequency level based on the audiogram. If it wasn't, believe me, I would know. Compared to my real aids the APP2 Hearing aid function gives much less gain. But it's better than not having the function at all when I want to wear the AirPods in public when someone might talk to me.
If you want to adjust the value, change the underlying audiogram data. Or move the "brightness" and "amplification" sliders.