TestFlight: internal and external testing
Essence
TestFlight is a way to distribute a beta through App Store Connect: a tester installs the TestFlight app, accepts the invitation and receives builds over the air, with no UDIDs or ad hoc profiles. The build is signed for the App Store — it's the same build you can later submit for review.
The rest of this answer, with code and interview follow-ups, is in the DevPrep app.
Read the full answer in DevPrep