It's on their security blog:
https://security.apple.com/blog/expanding-pcc/
"However, until today, those primitives have never been integrated into a comprehensive, end-to-end confidential inference pipeline capable of operating at global scale. That’s what we’ve done with PCC on Google Cloud, which incorporates PCC’s exceptional security and privacy properties at every stage, including the industry’s most comprehensive transparency guarantees that allow external security researchers to verify our privacy commitments."