Plus given the similarities between APIs/languages, the same team of developers could likely handle iOS, iPadOS, and macOS without significantly more work than

Plus given the similarities between APIs/languages, the same team of developers could likely handle iOS, iPadOS, and macOS without significantly more work than just iOS and iPadOS (assuming the developers are comfortable with all three platforms). The majority of the underlying libraries wouldn't necessarily need to be changed; "only" the UI layer and some of the APIs (which is non-trivial, but doable).