Every language has a passive form. But, what is it?

Let's revise the active form first:

Matteo eats an apple/ Matteo magia una mela (I am obsess with mele, I know...)

Matteo has eaten an apple/ Matteo ha mangiato una mela

Matteo ate an apple/ Matteo mangiava una mela

Matteo is the subject and an apple is the object of those sentences.

In the passive form, on the contrary:

An apple is eaten by Matteo/ Una mela e' mangiata da Matteo