How I'm reading it, they confusingly refer to 2 different bits of code as GetById here.

In the first instance they're referring to this method on the ShoppingCartController:

```public ActionResult<ShoppingItem> Get(Guid id)```

This is the method that they are unit testing.

In the second instance they're referring to a piece of code that isn't currently under test, this method on the IShoppingCartService

```ShoppingItem GetById(Guid id);```

As Niels says, this method on the IShoppingCartService is mocked/faked because it's outside the scope of a unit test.