Although Apple in the past has relied on Amazon S3 and Microsoft Azure to host its iCloud storage solution, but change is afoot. The first indications are now surfacing that Apple has moved [at least in part] to Google's Cloud Platform. It's not clear if Apple is exclusively on Google's platform or if it's using a mixture of competing solutions for the storage needed for all the images and other data that Apple users upload to iCloud.
The move might seem rather surprising considering that Apple and Google are rivals in the smartphone realm, with each representing one of the most popular mobile operating systems out there. An update to the iOS Security Guide made last month officially signaled that Apple had moved to Google's platform. Apple really wants users to know that Google can't access their data.
Each file is broken into chunks and encrypted by iCloud using AES-128 and a key derived from each chunk’s contents that
This means that even if Google did access a user's iCloud data all they would get are encrypted chunks of files. Apple's iOS Security Guide is meant to explain what the company does to ensure the security of iOS and user privacy.