> The linux version of swift replicates a large number of native mac frameworks using whatever mix of C and C++ gets the job done.

Note that this is changing, on Darwin and Linux alike. Foundation.framework is transitioning from being a Swift interface over a C / Objective-C implementation to an Objective-C interface over a Swift implementation (https://github.com/apple/swift-foundation).

The language still has native C interop, and is gaining the same for C++, so it can delegate stuff like crypto to existing solutions like BoringSSL, but Swift-native solutions are becoming more common.