As far as I could tell, if you create a Main.swift file, you can't just open that in XCode and start running it as an iOS/macOS application, and instead have to create a .xcodeproj/.xcworkspace through XCode, and add your Swift to the scaffolded project - this seems backwards to me.

(I then separately complained at all the steps it took to get it running without XCode, as I didn't want to be locked into using it)

Not op, but I think "doesn't play nicely" means does not work so you have to do it in other ways. This has been my experience as well, albeit it was couple of years ago.