Apple的TestFlight应用程序是否自动更新外部beta测试人员已安装的应用程序？

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 see from my iTunes connect control panel