I was surprised to see this. From the original keynote, it seemed like an iPadOS-only option, but this video made it clear that it applies to both.

There's still a lot of effort to keep subsystems and application code-bases the same - iPad OS I believe is mostly a sign that there may continue to be UX deviation (widgets on home screen, drag and drop between applications, split view multitasking, etc)

Each supported device has always had its own image, so I assume it’s not exactly the same codebase.

The only reason I know this is because the last time I had to upgrade iOS using iTunes - going from iOS 4 to 5, I had three separate devices that required separate downloads for my iPhone 4, iPod Touch, and iPad.

And Samba shares, which from my perspective is even more interesting.

Minor correction. Will support USB media in iOS 13.