Thank you very much, it's now clearer to me how it works.

Regarding the objects themselves, the way you describe them makes me think of Javascript objects & arrays: you don't need to know the exact shape of an input object, as long as it has the specific properties that you want to work on. Same for arrays: regardless of their content, the language allows you to use the standard ops like map() or find() (and it's then a matter of you providing adequate functions to access the contained elements)