For starters, the source code.
Apart from that, for the UNIX part, the source is available for quite a few components:
https://github.com/apple-oss-distributions
notably also the kernel
Strictly speaking, Apple can claim to ship a UNIX® operating system:
kokonokko1337 already said it was good enough to run LLMs, presumably RunSet isn't saying the source code is needed to run an inference server.