Yes just the language. iOS apps are built using UIKit or SwiftUI gui frameworks, neither of which have been ported to platforms other than macos or ios. This,

Yes just the language. iOS apps are built using UIKit or SwiftUI gui frameworks, neither of which have been ported to platforms other than macos or ios. This, and Foundation, are frameworks that allow you to write non-gui linux apps in swift (e.g. swift on the server) - and these could potentially be ported to windows as well.