TikTok movie/tv show clips

I’m wondering how/where people get their clips from to post on tiktok/youtube shorts, how can you clip a video from a movie or tv show that’s good quality and fits the iPhone screen? I can’t find a way to do this anywhere… I want to get clips from things I enjoy to watch and edit them to post on tiktok, and I don’t want to just take from YouTube shorts or other tiktok posts, I want to make my own original ones :) anybody know how this is done ? Do you need a computer 💻 for this