World War Z is technically a first-person peripheral narrator but it reads like first-person central narrator, and… we're getting ahead of ourselves, aren't we? Let's back up a bit and start with what it means to be a first-person narrative.
You can tell a story is in the first person when the narrative character is telling you what's happening to them. The easiest way to discern this fact is if the narrator uses the "I" pronoun when speaking. "I" equals first person about 99.6 percent of the time. Rounding down.
A peripheral narrator is a narrator who isn't the central character in the story being told. They stand on the sideline—the periphery —and tell you what they are seeing, like a sports announcer versus someone actually playing in the game. If you've read The Great Gatsby, then you've read a first-person periphery narrator. Nick Caraway sits on the periphery and tells you the main character's, Gatsby's, story.
But wait—aren't all these stories are being told by characters in their own story? Like, a central narrator?
Yes and no. It reads like it's a central narrator because each interviewed characters tells us his or her story. But remember that all these stories are being related to us through the Interviewer. The best way to remember this is to check out the opening of any section:
[I stand on the short with Ajay Shah, looking out at the rusting wrecks of the once-proud ships.] (4.2.1)
This opening paragraph lets us know the Interviewer is on the periphery, relaying to us what the central narrators—the interviewees—have told him. And since the central narrators aren't telling us, the readers, directly, we have to land on first-person peripheral narrator as this novel's narrative technique.