Yeah as the previous commenter said, e2e encryption just doesn’t allow anyone to access the data but the owner of the keys. E2E is prized because of this. There are two keys: public and private. If you and I are both using iMessage, you send a message to me that is encrypted on your device using your private key, and sent to my device using my public key. Only you and I can ever see those messages unless someone gets access to one of our phones.
Now, iCloud is backed up to apples servers. If you have iMessage backup enabled, it’s possible, and maybe even likely tbh, that Apple has access to recent messages. iMessage is also (potentially, but again in this case, I’d argue likely) susceptible to man-in-the-middle attacks. Because you need my public key for our communication to be decrypted, if you receive some else’s public key instead, they now have your messages and I don’t.
The DEA and FBI have both had documents leaked mentioning they can’t track or trace or unencrypt iMessage. The same is true for WhatsApp or any e2e messaging service.
Again, this is all contingent on not using iCloud backup. If you use iCloud backup, then the encryption keys used can be accessed with the proper authority. I assume (but haven’t looked into it) that Google is the same. If you don’t backup your e2e encrypted content, it cannot be decrypted without the private key only you have access to. Of course iCloud backup is enabled by default, so for the vast majority of Apple users, their messages and information are all available anyway so none of this matters.
In addition, iMessage uses a directory lookup to find the correct public key for your recipient. This information Apple does keep (I am unsure how long). What this means is that law enforcement (with a warrant) can see who and how often you are messaging. That alone is information we really don’t want people having.
So the moral is: if you don’t use backups for e2e encrypted communication, your content cannot be read externally. It’s just the way cryptography works.
This doesn’t mean that companies do not share information with law enforcement. There is a lot of unencrypted information Apple, Google, et al will share with government agencies when a warrant or subpoena is served. In addition to that, your phone provider will share information with them. In addition to that any SMS or MMS messages sent from any device will lack encryption and be easily discoverable.
Tl;dr: e2e encryption is secure, as long as you follow best practices and have an idea of how encryption works.