They don't 'tell' you, you press Command-B and it happens. When there are new architectures, you get a warning in XCode and press "Upgrade Project". Right now

They don't 'tell' you, you press Command-B and it happens. When there are new architectures, you get a warning in XCode and press "Upgrade Project". Right now my 'default' build is for armv7, armv7s, armv8, x86, x86_64. This setup required no effort on my part. I just created a new project.