With Bluetooth advancement (Bluetooth 5.0) hearing aids get more smart features, but in terms of design we can break the into 3 types. Behind-the-ear hearing aids come with a plastic case and a earbuds with silicone eartips. The case hosts electronic parts and also keep the hearing aids stay put over the ear. You can choose between sealed and unsealed eartips while sealed eartips allows earwax buildup it has a better noise isolation, unsealed fit on the other hand feels more relaxed.  In-the-ear hearing aids are custom-made to fit into your outer ears (like in ear monitors). Some are built with bone conduction technology.
Basically  canal hearing aids can be categorized to custom hearing aids to fit perfectly in to your ear canals and invisible hearing aids (you wear it and people don’t see it). Battery life and loudness level are major disadvantages of these hearing aids.