First person point of view is where a character uses first person pronouns, such as I or my. This means we get a direct line to the person's inner thoughts, with no filter. Third person point of view can be limited or omniscient. When it's omniscient, we know every character's thoughts. When it's limited, we know one character's thoughts at a time. In this case, the story is told from Peter's point of view first person, then Peter tells Cass's story third-person limited.
The overall point of view is Peter's first-person narration. However, in some chapter's Peter talks through Cass's character as he recounts flashbacks or reads from Cass's notebook. In these instances, Peter, the first-person narrator is lending his voice to Cass's voice. This technique creates an embedded first-person narrator: Cass's first-person narration embedded within Peter's original first-person narration.