Apple-centric? The libraries in iOS and macOS, for sure. However, the language itself can be used anywhere where GCC is available.

It's always been cross-platform; GCC has supported it for decades. Apple's Cocoa libraries are Apple-centric, but there have long been alternatives like GNUStep.

FWIW Objective-C predated Apple (or NeXT which adopted it and brought it to Apple) and was pared by Smalltalk.