Covariant data types: Why return type must be same as or child of its 'parent method' return type?

As this is not so famous concept, I will make a little intro.

Covariant return type of a method is one that can be replaced by a
"narrower" type when the method is overridden in a subclass.

So I can compile just fine this little program (as String is child of Object):

public class House {
    Object someMethod(){
        return null;
    }
}

class DogHouse extends House{
    @Override
    String someMethod() {
        return null;
    }
}

The rule is easy enough to remember, but I don’t understand it. My question is this:

Why can the return type in someMethod in DogHouse only be the same or child of return type in someMethod in class House? I hope the question is quite clear.

Or..(for example) why this code wouldn’t compile if I had put return type in someMethod in class House Integer for example? (String is not a child of Integer)

What is happening ‘behind the scenes’ so I can understand it?