I’m not an expert by I have worked with a hearing aid producer, though not directly with the signal processing. Roughly the thing is that hearing loss doesn’t just mean that you have for instance 30% reduced volume on your hearing. So just naively amplifying all sound won’t help you that much. What you need is to measure the hearing loss at different frequencies and then amplify those frequencies in the input signal. Now thats a very crude simplification and a lot more complex signal processing goes into the actual products based on things like making the signal source more clear for people with high frequency loss, since many people suffering from hearing loss will have issues in crowded spaces or conversations with multiple people because it’s not at easy for them to “tune out” notices unrelated to the person they are listening to. Then of cause there a a bunch of things you can do to try to isolate typical “useful sounds” compare to environmental sounds.

Hope that helps a little to explain the difference. This is also why you can’t really have a hearing impairment aid without doing the assessment which it sounds like Apple can now do with just the AirPods and an iPhone, because it’s never just “tuning up the volume”