I can see from my iTunes connect control panel how many users are on each version of the app I have in beta. When does TestFlight update those apps? Do they have to open TestFlight and manually choose to update? Does it happen automatically when they launch the app they are testing?
I can't seem to find any answer on Apple's documentation.
They have to manually update it. A push notification and email is sent to let the user know that a new version is ready to be tested, but it's up to user to open test flight and install it.