Why hearing aids are 96 frequencies. I get a detailed hearing test and then the audiologist adjusts the individual frequencies according to the results of what I can hear at those individual frequencies.
So in answer your question it’s done by the audiologist
I’ve been working with the same guy for 8 years and I think there’s a bit of an art to the adjustment
You should be able to adjust it (tweak it) yourself for the particular environment you’re in.
If certain frequencies are coming in too loud you lower them, if too low you raise them.
You don’t need an audiologist.