You're mixing up the terms.
There is the protagonist, and there is the narrator. Narration has perspective.
The narrator is the voice in which the book is told. If the story is told using "he/she/they" and not "I," it's third-person. This narrative voice (perspective) can see into everyone's thoughts (omniscient) or only one person's thoughts (limited).
If the narrator is using "I" and the story is only about what the "I" narrator sees, knows, and experiences, that's first-person. The story is told from one person's perspective.
The protagonist is the main character of the book. A book may have multiple protagonists (see A Song of Ice and Fire), primary and secondary protagonists (the Harry Potter series), or one protagonist.
The protagonist does not have to be the narrator. The narrator does not have to be the protagonist. You can have a first-person narrative, like The Great Gatsby or Song of Achilles, where the person who is telling the story isn't the most important person in the story. It's still a first-person narrative.