expandable.kt

 val bigTextStyle = NotificationCompat.Builder(this, "first")
                .setSmallIcon(R.drawable.ic_launcher_foreground)
                .setContentTitle("Hello There")
                .setContentText("General Master!")
                .setStyle(
                    NotificationCompat.BigTextStyle()
                        .bigText(getString(R.string.big_string))
                )
                .setPriority(NotificationCompat.PRIORITY_DEFAULT).build()

//Show the notification with the help of NotificationManager, Unique ID ->3
nm.notify(3, bigTextStyle)