How is "se" used in the context of passive voice?

Question

How is "se" used in sentences with passive voice? For eg: The book is being read by me, Pizza is being delivered by her etc.

Answer 1

Se cannot be used for a true passive, hence it's often called the fake passive for SSL students, because it's actually in active voice.

To take your sentences

• The book is being read by me.
  • active Yo leo el libro.
  • passive El libro es leído por mi.
  • “se passive” Se lee el libro (*por mí)
  • “se passive” passive El libro es leído por sí mismo.

Notice the issue is primarily in declaring agency. Because literally you are saying that the book reads itself, the literal passive (re)construction would be the book is read by itself, which excludes the possibility for other agents. Same with your pizza example.

• The pizza is being delivered by her.
  • active Ella está entregando la pizza.
  • passive La pizza está siendo entregada por ella.
  • se passive Se está entregando la pizza.

You can add many other details to the last bit, but not who is delivering it because adding por ella would, at best, be a slightly awkward way of saying she made the pizza that's being delivered, but not be interpreted as an indicator of agency. You have to leave it as a sort of anonymous passive (basically, English passive without the "by" prepositional phrase).