Due to some unique quality and regulatory requirements in our organisation, we need to be able to download a .ipa build from App Store Connect, in order to be able to compare it against a locally built binary. I appreciate this is an unusual requirement, but it is non-negotiable.
Clearly there is no way to do this via the App Store Connect web interface, so I've been looking at the App Store Connect API. However it is ambiguous from the documentation, whether it is possible to use this to download an .ipa build.
Does anyone know if this is possible, and how one might go about doing it if so please?
Apple does not provide any way of retrieving an upload app/IPA. It's neither possible through App Store Connect nor the App Store Connect API.
You can only check entitlements and additional build information in the App Store Connect interface. For that go to App Store Connect > My Apps > Activity > All Builds > Click on the build
(Fastlane does also not provide any way, because Apple does not expose the data. It uses the "web API", i.e. has access to more information than the "standard" App Store Connect API)