There are 2 repositories under private-frameworks topic.
Headers and SPM linking for /System/Library/PrivateFrameworks/ProtocolBuffer.framework
An example project using the private BatteryCenter framework, available on iOS and watchOS
Properly patched iOS 10.2 SDK (private framework tbds)
Example of using the private Persona iOS frameworks to create a monogram
Bringing back the pre-watchOS 3 Digital Touch.
Headers and SPM linking for /System/Library/PrivateFrameworks/ProtocolBuffer.framework