The dots on the app bar notification component display when there is a count that is greater than 0 within that parameter.
So you can add a new collection to your database and call it “Notifications”. Add a one-to-many relationship with users so that a user can have many notifications but a notification belongs to 1 user. Then, when a like button is clicked, create a new notification for that user.
I add a “Read” true/false parameter to the notification so that when the notification has been read, it removes the red dot.