Um iCloud-Daten speichern zu können, benötigt Apple Server. Sehr viele Server. Und die hat sich der Konzern bis dato bei Amazon (über Amazons S3-Service) oder Microsoft (Azure) geholt, nunmehr nutzt Apple auch Google-Server.
iCloud stores a user’s contacts, calendars, photos, documents, and more and keeps the information up to date across all of their devices, automatically. […] Users set up iCloud by signing in with an Apple ID and choosing which services they would like to use. […] The service is agnostic about what is being stored and handles all file content the same way, as a collection of bytes.
Each file is broken into chunks and encrypted by iCloud using AES-128 and a key derived from each chunk’s contents that utilizes SHA-256. The keys and the file’s metadata are stored by Apple in the user’s iCloud account. The encrypted chunks of the file are stored, without any user-identifying information, using third-party storage services, such as S3 and Google Cloud Platform.
Die Daten sind verschlüsselt, also alles gut. Wem’s trotzdem zu unsicher ist, der kann ja seine Daten selbst nochmal lokal am Mac verschlüsseln bevor sie in die Cloud geladen werden.