Earlier quoted context omitted.
iMessage has end to end encryption and supports multiple devices. Each device uses its own keypair and when a message is sent, it is sent encrypted for each of the recipient's devices. https://www.apple.com/business/docs/iOS_Security_Guide.pdf
The hard part is how do you manage a list of trusted devices. The thing which is really sad is that there are no end to end encrypted group chats.
e2e is in beta still, but they have key list management/verification in place already.