The build status has a green check in Xcode Cloud, but TestFlight see no builds and suggest that I submit one. Then it provides a link to an incredibly unhelpful list of upload tools, at least one of which suggests creating a developer account!!!
Xcode Cloud has a build, TestFlight doesn't see one
I found a video showing that you need to archive from Xcode. Why that wasn't the first thing you see when deploying, I'll never understand.
Now, I'm trying to get my wife's phone to be a test user, but it's an old phone using IOS 15, so it's crashing when trying to accept iCloud terms and conditions. Such fun!