LG v20 User Manual page 42

The Basics
40
Using Direct Reply
When a Direct reply message notification appears on the Second screen,
tap the Expand icon
You can immediately compose your reply message in the Direct reply
window without closing your current app, or you can tap the Direct reply
window to open its corresponding app. This feature is available on Google
and other apps such as messaging, Gmail, and Hangouts.
NOTE
then tap REPLY to reply to the received message.
Messaging, Hangouts and other apps support the Direct reply feature. However, not
all apps support this feature.