Apple iMessages are encrypted end-to-end. That means the messages you send on the platform can only be read by you and their recipient. Apple can't read them and because it does **t have the encryption key. It can't decrypt them, even if the authorities request it.*
However, before a text message is delivered on the encrypted iMessage network, in fact, when you enter the contact number, according to a report in The Intercept, there's a check that happens in the Apple Message app. It determines whether the message will be delivered via SMS or through iMessage. Sometimes that check gets logged and, yes, stored with Apple. Read more...