It sounds like you could really use the book "Characters and Viewpoint" by OSC. He goes into great detail about this. Let me try to sum it up in my own clunky way.There are three persons...first, second, and third.
First is when the narrator is the main character and tells the story for himself. "I did this and that."
Second is when the reader is the main character and there is an actual or implied "you." This is usually reserved for the imperative. Instruction manuals and cookbooks, for example. You may also remember the "Choose Your Own Adventure" stories, though.
Third person is when the narrator is removed from the story in some way and tells it for the characters. He/she/they/it.
Now, when you choose third person, it gets interesting. How much does your narrator know? Where are his eyes? What does he see/sense?
An omniscient narrator knows all, sees all. This is becoming less popular but can still make for an interesting story when well done. You can choose what information to tell and withhold as an omniscient narrator, but the more you make it obvious that you are withholding information the more intrusive the narrator is to the story. The more it is obvious that you are reading a story rather than experiencing it.
Third person limited omniscient means that the narrator sits in one character's head and tells the story from there. This narrator has access to all the thoughts, feelings, and senses of that one character and it is considered poor form to purposefully withhold any of that if it's important. (That isn't to say you have to infodump their head on the first page.) The advantage of third person limited omniscient is that you can make it feel like the reader is experiencing the story with the character. I find these kinds of stories particularly enjoyable to read and to write. You can always use more than one viewpoint character, but you need a chapter or at least a scene break to switch viewpoints and then you have to be completely honest with the second character's point of view.
Now, who is your point of view (POV) character in third person limited omniscient? Anyone! The idea of main character and POV character are different. You can have a sidekick or an interviewer tell the story, but when you do that you have to understand that the story teller is an important character as well and that we will be deeply entrenched in their heads. We need their emotions, thoughts, and attitudes about the story.
So if you have a confessor going in to talk to a man who has been in prison for 40 years, you can do that from the confessor's POV without problem, but let's get inside the confessor's head and find out what he thinks as he's interviewer your prisoner.