At my company I am the pioneer of trying to move our macOS and iOS codebase closer together, by creating a unified UI layer, which can be used on both platforms. On the 2 recent WWDC made big steps into that direction, in 2025 we got the iPad Windowing System, which makes the iPadOS closer to macOS. This year we have the App Adaptability, where our iOS app can support different screen sizes, even going to sneak into the macOS area (except the EU :P), where iPhone Mirroring can translate our app into a(n almost) macOS app. I will hop through what changes are required for support this transition, and also on what can go easily wrong and how to fix it.