When the customers access your online store, they will see the live FB Messenger Chat as configured from the app integration. The customers can see the live chat popup only on those page(s) that you have selected from the app integration.
The customers can see the live chat popup only on the desktop and/or mobile device as selected from the app integration. If you have selected Messenger for the “Chat Type”, the customers will be redirected to the new browser’s tab to start a conversation with you.