The core language has been open source for a while. Windows support was added a few days ago. It will not allow you to develop iOS apps on Windows, as the tools to do that are part of Xcode, which is not open source and not cross-platform.

This announcement concerns Swift System, which provides a wrapper over UNIX functions like `open'. Since those functions come from C, they do things like using integers to represent files (``file descriptors'') and setting errno to indicate failure. System wraps them to make them into proper Swift functions that throw exceptions and make full use of the type system. It has been open-sourced and Linux support has been added.

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.

Not iOS apps, but windows applications[1].

[1] - https://swift.org/blog/swift-on-windows/

You might want to check out https://github.com/compnerd/swift-win32 as well