Dyld shared cache
Since iPhone OS 3.1, all system (private and public) libraries have been combined into a big cache file to improve performance. The original files are redundant and thus eliminated from the system. If you're looking for binaries or libraries inside of /System/Library/Frameworks or /System/Library/PrivateFrameworks (or other directories) and can't, this is why. OS X also uses a shared cache. Unlike iOS, OS X ships with the source binaries still on-disk, particularly so it can be updated with update_dyld_shared_cache. The cache is only vaguely documented in dyld man pages.
== Extracted dyld_shared_caches iOS 10.3.1 ARM64