The source code for Apple's NSObject
is contained in the Objective-C runtime package on Apple's open source repository site.
https://opensource.apple.com/source/objc4/objc4-706/runtime/NSObject.mm
The whole Objective-C runtime package can be downloaded from https://opensource.apple.com/tarballs/objc4/objc4-706.tar.gz
Looking back through the previous versions of the runtime package on the site it appears the NSObject.mm
file was added in the objc4-532 package.