How does a property of child components change without any communication between child and parent component?

How does the property of child components change without any communication between the child and parent component? Like magic, the property of child components is manipulated without parenting to the child. Can someone explain to me how this happens? Please see the code here, https://plnkr.co/edit/ucQ47vTBSOyBv8jr1qEp?p=preview I am updating the value of the form control with

SetFormByPatchValue()  {
    this.formGroup.patchValue();
}

      

        
        
        
      

    

The above code will manage the properties of the child component.

Example: 2 Here is a link to another plunker . You can see that I have added another date component that does not change its value when edited.