Siri can announce calls and notifications from apps like Messages on > Siri & Search > Announce Notifications, then turn on Announce Notifications.
Tap an app you want Siri to announce notifications from, then turn on Announce Notifications.
For some apps, you can also choose whether to announce all notifications or only time-sensitive notifications.
For apps where you can send a reply, like Messages, Siri repeats what you said, then asks for confirmation before sending your reply. To send replies without waiting for confirmation, turn on Reply Without Confirmation.