https://www.theverge.com/2018/2/26/17053496/apple-google-cloud-platform-icloud-confirmation

Apple has confirmed that it uses Google’s public cloud to store data for its iCloud services in its latest version of the iOS Security Guide last month, as spotted by CNBC. Reports that Apple relied on Google’s cloud services surfaced in 2016 but were previously never confirmed.

Apple had previously used remote data storage systems provided by Amazon Web Services and Microsoft Azure. Apple’s edition of the iOS Security Guide in March 2017 still listed Microsoft Azure instead of Google Cloud Platform.