How can I use the Notification Dots feature?

After Android 8.0, Google has made improvements to Android's notification functions by adding a new Notification Dots feature. It’s a small loop point above an application icon which only appears when the app has unread notifications. When you press and hold the application icon, the notification can be quickly previewed in a “bubble window”. When these unread notifications are deleted in the notification center, the marker points on the application icon will disappear.