Ramadan? muslim what does mean in the Quran to be people of the Book?

I am not against islam but reading about islam.. it says it's a religion that is superior to others, so the people of the book are inferior that's the reason they were obligated to pay yizia or to fight under an islamic state? or what does mean in many senses to be people of the book.. a christian and jewish? and honestly.. I don't get why do some muslims say catholics are not people of the book.. because we are trinitarians, I guess prophet muhammad knows about us before those unitarians or another sect because God explains him. he wasn't a trinity in the Quran as far as I understood.. so I guess the prophet knew about catholicism and what we believe but returning back what does mean to be people of the book and who are them and can you identify them nowadays?