What are some commonly used sites that are P2P?

I have this paper due and it's about the legality of file sharing. My teacher said that there are some sites that people don't know are file sharing like youtube but it has a loop hole with the copyright laws. I know about Hulu and I also know about yidio but I don't know if it is legal so if you could answer that it would be appriciated. So my main question is what are some commonly used sites like youtube, hulu, and yidio that are P2P and are they legal?