Re: But you're an Apple user.
I know a few Unix bods who use Macs (they see macOS as a version of BSD with a nice GUI, which it is, IMO) who would disagree with the apparent assumption in your second sentence.
Personally, I think this is a stupid lawsuit. Apple are selling a service, and they are selling the integration between that service and the devices using it. They are not selling a backend. It's like suing a PC manufacturer because they didn't tell you that the parts were made by Intel, Gigabyte, Crucial and Samsung and not the manufacturer. When you buy a device or service, you are paying for the components themselves (be they hardware, software or other services), and you are paying for someone to put them together, and support them (you are also paying for dozens of other things like warehousing etc, but that's beyond the scope of this comment).
I don't understand what they hope to gain by taking this action. Even though Apple are likely not storing all the iCloud data on their own infrastructure, any contracts the users have are with Apple. Apple can still have action taken against them even if it's one of their providers that fail. It's still Apple's reputation that will be affected. Apple would need to take any action required against their providers.
If it's merely an objection to Google, Amazon or Microsoft having their data, I think they are in for a rude awakening. A *lot* of companies (and a *lot* of household names) have outsourced a lot of their IT infrastructure to Google, Amazon or Microsoft. So it is likely that most of us have personal data on one (or all) of those cloud systems. I read somewhere a while back (so it may have changed) that Netflix uses AWS rather than maintain it's own infrastructure, so, yes, Amazon are providing computer power for a major competitor.