See e.g. https://developer.apple.com/documentation/network/creating-a... where the logging output makes it clear BoringSSL is what is used.
Or comments such as: https://github.com/apple-oss-distributions/Security/blob/rel...
Unsurprisingly, given BoringSSL doesn't have a stable API (yet alone ABI), it isn't exposed as a system library.
Seems like they use BoringSSL on their open source distributions, but their own library on their own platforms: https://forums.swift.org/t/native-implementations-and-boring...