The iPhone hearing aid control screen is reachable almost instantly via triple-click of the side button. From there you can control volume, program, and equalizer if available, and see battery level (interpretation of battery level for zinc-air batteries is a whole separate discussion). It’s faster than even a good hearing aid app like ReSound’s. And it’s certainly better than a buggy app, and judging from this forum some hearing aid apps are pretty buggy.
You can also turn on Live Listen from that screen. It turns the iPhone into an assistive listening device AKA remote microphone, bringing the mic closer to the person who’s talking.
MFi includes the ability to make phone calls where received sound is sent to your hearing aids, and you talk into the iPhone mic. You might think this is obsolete now that aids, even MFi aids, have hands-free calling using the hearing aid mics to pick up your voice. But for phone calls the iPhone mic has better noise cancellation than the hearing aid mics, so some of us still prefer to use it instead of hands-free.