I wouldn’t assume that any iOS engineer would be jumping all over the opportunity to make Android or Windows apps just because they can do it with Swift. They have their (typically pretty lucrative!) niche and are happy living in it.
AppKit is still dominant on macOS (despite the addition of Catalyst), UIKit is the chief framework on iOS/iPadOS/tvOS/visionOS, SwiftUI is native on watchOS and mostly wraps AppKit/UIKit on the other platforms. macOS is oriented around KB+mouse usage, iOS/iPadOS/watchOS are touch-dominant, tvOS is made to be remote-friendly, and visionOS is oriented around eye navigation and hand gestures.
Apple claimed they were running OSX back at the iPhone launch, but it was truly very stripped down to accommodate the underpowered hardware, and they've only made incremental, very targeted expansions in the last 16 years.