It depends, for us we write some wrappers in Kotlin and Swift, but this is irrelevant to team performance - this could take time, but 90% of the time work is done in typescript. You want to have some folks that can do hacking on native level.

We built our own since there are much better frameworks on native side - asyncdisplaykit on iOS and Litho on Android. This library claims 60fps without actually measuring it or using very simple layout. We are doing messenger and layouts are really heavy for rendering and rn have it’s limitations, more importantly we reuse lists between screens, we have our own micro react inside that renders mostly static content to json and send it to native side. This is overkill for some, but not for us. We would have done the same without RN anyway.