Yeah, I've been wearing various different models for about 30 years, I'm 42 now. I've been blogging about is for about 13 years now, what started out as a blog has turned in to more of an informational site type of thing: https://www.hearingaidknow.com/

Modern hearing aids mostly avoid feedback by having a cancelling algorithm built in to the software - they are very effective these days and you would barely ever hear a squeak. But also, there's a type of hearing aid called receiver-in-canal/ear (RIC or RIE) which has the sound output in your canal and the microphone in a small unit behind your ear and because they are far enough apart feedback does not occur. I'm wearing GN ReSound Linx 3D RICs at the moment, they are amazing tech, I can remote programme them from my iPad and stream phone calls + music directly through my hearing aids.

As someone said in another comment it is friction holding them in, a moulded aid in the canal does fit snuggly, which is what I have, but for less severe hearing losses many people wear a round one-size-fits-all flexible rubber tip in the ear, it does press against the canal enough to hold it in place but is super comfortable.

I've never once had a hearing aid pop out of my ear and I've worn them doing all sorts of sports activities.

They're shaped to your ear canal. It's mostly surface area and friction holding them in. They really don't pop out that easily. A motorcycle helmet will do it, though.

> (Also, falling out.)

More or less the same way headphones do.