That question had been floating around my mind for weeks. I searched for an answer found this four-month-old blog post.
I have a lot of camera apps on my phone, yet not one of them has an extension. The little three dot icon for extensions does not even show up in my photos app. So it’s great that Apple added this ability, but it seems that developers do not want to take advantage of that. It’s a bummer for the user. But perhaps extensions often go against the interests of developers who want users inside their apps, not using an extension outside of it.