The simplified rule: if you are refering to a specific person or thing, use “il/elle”. Otherwise, use “c’est”.

• If the video is wonderful, you should say: “C’est magnifique” because the video in which the dance is beautiful and the girl dances well is globally wonderful. You don’t point anything being wonderful, but the whole thing is.
• If people already know that you are specifically talking about the video, you can
  say “Elle est magnifique”. Example: someone comments the video and say “C’est une très belle vidéo”. You could answer “En effet, elle est magnifique”.

• If the girl who is dancing were to be wonderful, you could say without any context that “Elle est magnifique”. Here you are implicitely talking about the girl because the two following conditions are met:

  • you wouldn’t have said “C’est magnifique” to talk about the girl, because she’s not an object. It is the same as in English when “It’s wonderful” for a girl sounds strange.
  • the context doesn’t say anything about what you are refering to, it means you point something/someone out.

• Note that you could say “C’est une fille très charmante”.