There are many areas on how iOS and macOS are influencing each other. With the upcoming iOS 13, macOS 10.15 and Marzipan, it will be even more important. Jason Snell writing for Macworld in April 2019 issue:
it seems pretty clear to me that macOS is in the process of turning into something that’s got a lot more in common with iOS than it does today. Every time you use an iOS device and your Mac and notice that the two platforms approach the same situation from different directions, consider that there’s probably someone at Apple who has been pondering how to unify that behavior for some time now.
I would add this: in the process of porting iOS apps to macOS via the Marzipan technology, both platforms influences each other. But the platform who will benefit most is iOS. Here is why.
iOS apps, in general, are lacking features compared to macOS counterparts. Consider the rumoured porting of Music.app to the Mac as exposed here. In the process, I expect Apple to beef up the Music.app on iOS. The point of reference here is iTunes which has more features than the Music.app has on the iPad. I’m thinking about smart playlists creation and management for example. The ported version of Music.app will be a more feature complete application than we currently enjoy these days. This is certainly my hope.
Another example of macOS influencing iOS is tabbed documents. This feature is rumoured to be coming in iOS 13 too. Moreover, I really think it could be at the centre of the home screen redesign. What if a concept of workspaces would allow a user to arrange apps together on the home screen. Tabs would represent workspaces. This could be one of many examples of tabs usage.
As you can see, the biggest beneficiary of this long transition will be the iPad version of iOS.