In this podcast, Scott Davert shows us the features available with MFi supported hearing aids or Cochlear Implant processors from the perspective of a VoiceOver user. MFi compatible hearing aids come from many manufacturers and there are numerous models supported. You can see Apple's official support page which gives a fairly accurate account of which hearing aids have MFi support. However, this article has not been updated since October 2018, so anything newer than that will not be listed on this page.
For more general information about hearing aids from a blindness perspective, you can listen to this Accessibility Moving Forward podcast featuring some familiar voices to AppleVis.
Finally, there is a mailing list for blind hearing aid users that you can join by sending a blank email to firstname.lastname@example.org.