If you want to hear a message that is sent to you, Siri will dictate the content of the message. You can repeat the message, send a reply or even call the sender, simply tell Siri what to do.
When replying to a message, Siri will ask you what message you want to send. Siri will check the message with you and then you can either check again, change the message or ask Siri to send the message.
If you want to write a message, tell Siri what you want to do. For example: ‘Send message to my wife’. Then tell Siri the message which it will then repeat back to you before asking for confirmation so it can send the message.