CF functions toll-free bridge the CF types to their Foundation counterparts and invoke the corresponding Objective-C API, which is either implemented directly in Swift, or is a wrapper around the Swift function.
An Apple employee on the Foundation team posted an example of how calling `CFCalendarCreateWithIdentifier` works here: https://forums.swift.org/t/swift-foundation-now-available/73...
I'm sure it's still a work-in-progress, but their definitive goal is for the OSS swift-foundation codebase to be the same as what ships in their OSes, which was never the case with swift-corelibs-foundation.
Edit: I found a conference talk by some Apple folks that goes into more detail https://www.youtube.com/watch?v=wn6C_XEv1Mo
Going forward, the bedrock of both the OSS package and the platform [Core]Foundation.framework will be swift-foundation. This is distinct from SCF, which was a novel Swift wrapper around CoreFoundation.