Read Cult of Mac's overview of the new GarageBand update for macOS. I think this is essentially adding the feature to the Mac version of GarageBand that Logic added a few months back. I played around with it for a few minutes last night, trying to sync a project between the Mac and iOS version of GarageBand. Unless I am missing something, this workflow runs into all of the same issues as the Logic feature that I wrote about when it was released. The process is not direct as you still have to manually prepare the file for syncing and create a duplicate copy whenever you go from Mac to iOS or iOS to Mac. And I really wish I could edit the audio on my iPad too. My iPad Pro is powerful enough!!!