Why does the original text: iMessage appear in the typing box when I send a message to this friend? He also sent an audio.

Amr is the suffix of all recording formats, which means that your friend sent you a recording file through MMS. Because you have turned on the imessage function supported by Apple, it can support the mutual transmission of various files between Apple devices, so if you don't need these functions and just want to send and receive short multimedia messages, you can turn off the iMessage function according to the method provided by the above friends in the information in the settings. I hope it helps you!