I know internally they use an IPsec implementation written by Rust (I think in the iCloud infra). Heard this from an ex-Apple engineer Ben (forgot his last name) that did a wonderful presentation of Rust from first principles. He said that it was hard to get people in on Rust when most would argue for Swift.

Edit: This is the guy: https://rustcurious.com/course/

Some stuff was discussed at Meet with Apple security event a few months ago, and the talks on FoundationDB rewrite, or why Swift Embedded subset came to be.

However I miss them actually having had one of those 15 - 30m WWDC sessions, where they could have gone a bit deeper into the keynote examples