Last I checked they were phasing out their own DCs in favor of cloud-provided services. Though it's been a while since I have heard anything about it, so maybe those plans got canceled. It could have also been phasing out those DCs for only the specific services and not all services. My details on the whole thing are fuzzy at best.
As far as I know, everything iCloud and Apple Intelligence runs off of their own data centers if you happen to live "near" one, but you could still be using AWS and/or Google as well.
I live near the Danish Apple data center, and pretty much all my iCloud traffic goes there, with a small fraction (<10%) going to Stockholm, which has both AWS and Google data centers, so I assume they're using both for geographical redundancy (erasure coding)
It gets a bit more fuzzy once you start moving into Movies/Music/TV/Billing/whatever as well as their backend services for the store and monitoring.
Got it, yea my inside sources were as they relate to Siri and that was a decade ago at this point.
If you live in a region where they operate their own data centers, you will be running on Apple data centers. If not, you're running on a mix of Google Cloud and AWS (IIRC). They used to use Azure as well, but I think that's no longer the case.
In any case, your data is encrypted (by Apple) before being uploaded to Google or AWS, and only Apple has that key. Whatever E2EE encryption you use will be applied on top of that.