[kartickv]

It's not "one UI" as much as not reimplementing parts of the UI that don't change between macOS and iPad. Simplenote, for example, has a two-pane view, with a list of notes on the left and the selected note on the right, with a toolbar above the note. Don't reimplement the entire UI, implement only the diffs.

Like responsive web design, where you implement only the diffs between the phone and PC layouts of your site, while reusing things that don't change.