It depends, most of important and heavy lifting stuff in Apple platform, aka Core frameworks, are written in C, for example a quote from Core Text documentation [1]:

> Many of the low level libraries in OS X and iOS are written in plain C for speed and simplicity.

And don't forget that OSX/iOS are based on Darwin BSD, where most of heavy lifting is done by programs written in C.

- [1] https://developer.apple.com/library/mac/documentation/String...