I'll try to show you both working together: below, an component with id: foo has declared a Bar instance with a binding to foo's fooInt property.

We don't know or really care if it's property or alias, just that we can and do bind to it.

id: foo;
property int fooInt : 0;
Bar { barInt : fooInt; }

If we take a look inside our Bar.qml - we are using an alias to expose externally only the features/properties we wish, like so:

Item { 
   id: root;
   property alias barInt : barInternal.number;

   SomeOtherComponent {
      id: barInternal;
      property    int number : 0;
      property  color  color : "red";
      property string string : "";
   }
}

Now if I we go back and look at foo where the Bar instance is. We fully understand that is an property binding to Bar's aliased property. It still behaves exactly as a Bar property and you can control it as such.

Bar users only need to know Bar has an int property "barInt".

Bar { barInt : fooInt; }