Basics

Where to find Foundation.h:

cd /Applications/Xcode.app/
find . -name "Foundation.h"

./Contents/Developer/Platforms/AppleTVOS.platform/Developer/SDKs/AppleTVOS.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h
./Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h
./Contents/Developer/Platforms/WatchOS.platform/Developer/SDKs/WatchOS.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h
./Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Versions/C/Headers/Foundation.h