Nowadays, close to none!
If you start your iOS project with both iOSdesignExtensions and iOSKit (both of them are free on github) you most likely won’t need to write a single declare.
Almost each time I write a declare for UI stuff, I add the feature in iOSdesignExtensions, so that the whole community can benefit from my knowledge and experience with iOS.
I have also contributed to iOSKit for more advanced features such as sending SMS, displaying a menu and so on.
To be fair, just like any other system, you need to know how it works to be able to make an app for it.
When I started making iOS apps, I was an Android user. My first app was rubbish because I wasn’t accustomed to Apple devices. After purchasing my first iPad and iPhone, my second app became a leading player in its market.