Yes. The obvious universal API that targets big screens and small, touch or mouse is Qt. It started as a desktop framework, was made touch-aware to be used Noki

Yes. The obvious universal API that targets big screens and small, touch or mouse is Qt. It started as a desktop framework, was made touch-aware to be used Nokia and BB10 for phones, powers KDE 5 and I'm hopeful Ollie Paranoid will provide an update in a few weeks re: Plasma on postmarketOS. Now a company with Apple's resources should surely be able to reintegrate the Cocoa Touch hard-fork back into a common modernised Cocoa, which by now has a 30 year legacy from NeXTSTEP. Naturally you'll still need to customize a UI for Mac or iPhone but a unified toolkit is entirely possible.