iOS Question Is there a way to sell Apple iOS apps through a channel other than Apple?

No. Not to sell at the point of download.

You can provide access to an app for 120 days with Firebase Distribution. I use it for testing. It is easier than Test Flight.

Once the user has installed the app. I believe they can keep using it. Monetising it would have to be done via another route.
I've put a STRIPE library up on the forum which could be used, I think.

Thanks for the tip. I will take a look at it.

For testflight I believe it was necessary for the user to submit their UDID to me before they can use the app.

Is this the same if I have thousands of users of my app?

You have a good point,yes you still need to have the UDID of the users phone with Firebase so you are restricted to 100 phone ids per year. You need a store certificate which is not limited, but you cannot upload that sort of app to firebase.

So this would not work for your use case.

You don't need users UDID but their email if they install with TestFlight.