MacOS has posix_spawn. See https://developer.apple.com/library/archive/documentation/Sy... (yes, that’s an iOS man page. MacOS has the call, too, but I couldn’t find the man page online and it looks identical to me)

I don’t know how they implemented it, though. Under the hood, it could do the equivalent of a fork/exec pair.

XNU's posix_spawn implementation is not fork/exec-based. It does roughly what the API suggests it would do.