Hearing aids are probably more high-tech than you think. They are tiny. They have to go through a lot of trouble to prevent feedback. They need a long battery life. The good ones match their amplification to the inverse of your hearing loss (amplifying only the bands where you don’t hear as well).
[NotionSunday] put together a hearing amplifier project that probably doesn’t hit many of those design criteria. However, thanks to a 3D printed case, it looks pretty good. The device uses a dual opamp to boost the output from two microphones and feeds it to a conventional headphone.
The device is wired point to point, and is, perhaps, pocket-sized. The opamp circuit is simple. We might have considered an LM386 or some other integrated audio amplifier block to get better performance without blowing up the parts count.
Technically, devices like this are not really hearing aids, they are “personal sound amplifiers” and the FDA warns people not to substitute them for hearing aids. If you want a proper hearing aid, you might check this out, but you are going to need big ears since it has an Arduino inside.