In passive sentences, we put the patient at the beginning of the sentence to give it more emphasis: 400 films of Shakespeare’s plays have been made
We are interested in the films, not in who made them
Often we do not say who did the action in passive sentences because this is unknown, obvious or unimportant: He is better known as Jackie Chan ...
... by people generally - this is obvious
However, we can say who did the action, using by before their name: Avatar was directed by James Cameron
To form the passive voice, use [be] + [past participle]