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. Apple relied on Google's cloud services since 2016, but its never been previously 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's Cloud Platform.
The new edition describes its iCloud service: “The encrypted chunks of the file are stored, without any user-identifying information, using third-party storage services, such as [Amazon] S3 and Google Cloud Platform.”
It is good to know that even though the data is hosted by Google, it is probably not accessible or actionable in any useful way by Google. Google has a responsibility and every incentive to respect the data of its enterprise customers – especially with a deal like this, which may be worth hundreds of million dollars.
The other iCloud development worth noting is that Apple reportedly plans to store encryption keys for its data service for users in China within that country itself. It remains to be seen if this will affect sales of the company's product in China, now that its data is more susceptible to oversight by government authorities.