From iPhone Development Wiki

Private frameworks are frameworks which Apple intends for use only by Apple's own apps. Private frameworks are more unstable than public frameworks between iOS version updates, but many of the interesting features of iOS are in the private frameworks. They are more likely to change between iOS versions, so make sure you check which version of iOS your code is running on.

Attempting to use private frameworks in an App Store app will result in rejection of that application.

List of Private Frameworks

The following list includes all Private Frameworks as of iOS 16