To open the messenger in Instagram, we tap on the icon in the top-right corner.
If there's a previous message thread as in this case, you tap on it to view previous messages and send new ones. You can send text messages, tap on the heart icon to send a heart, or tap on the camera to send a photo or video.
Tapping on the camera icon causes the keyboard area to be replaced by a camera viewfinder. You can tap on the circle icon at the bottom of the viewfinder to take a photo, or tap and hold to record a video.
Once you've captured a photo/video, the circle icon becomes a send button. You just press that to send the message.