This is a UI-centric take.

There are lots of internals that could/should be shared between apps. Recently relevant examples: a customized WebRTC stack, a websocket client with extra authentication and delivery rules.

FWIW, my take on this has been to embrace Conway's law: if you have enough shared work to form a team around it, then break off a c++/whatever library and a team to maintain it. But if the hope is just to save 200LOC here and there, just bite the bullet and write it twice.

Big thanks to whoever pointed out Kotlin Native: seems interesting enough to poke around with.