Instead of providing the same UI that works badly on every device, SwiftUI interprets each concept differently according to the platform.
For example, a Toggle in an iOS and iPadOS app looks like a on/off switch, while on macOS it's rendered as a checkbox. A Picker has is scrolling wheel on iOS but it's shown as radio boxes or a drop-down menu on macOS.
Navigation is also the same. On iOS, a NavigationView offers the drill-down navigation common to iPhone apps, while on macOS it creates a split master-detail interface.
Each implementation does so by using the native UI framework of each platform under the hood, UIKit on iOS, and AppKit on macOS. I don't know how UI development works on Windows, but my speculation is that a SwiftUI implementation would work the same way.