They backup the private key to iCloud unless you manually disable backups. So even though iMessage is advertised as E2E encrypted, for the vast majority of users, Apple can read each and every message.
(And even if you disable backups, Apple can still read most if not all of your messages, because the persons on the other side of the conversations have not disabled backups)
(And even if you disable backups, Apple can still read most if not all of your messages, because the persons on the other side of the conversations have not disabled backups)