Is that how they are handling any of the Swift UI work for Arc (their browser)? I think they are 100% focused on bringing the Swift compiler etc. over and leaving all UX to the Windows UI (WinUI lib?) so that they have a native experience on macOS and Windows.
You have Swift-NIO for cross platform form networking, OpenCombine as an open source implementation of Combine, and Iām not sure what you mean by file system but you can read/write files without needing any library other than the cross platform Foundation implementation