If you’re looking to add a video from YouTube or Vimeo inside a post, you just need to copy and paste the link of the video in a specific area of your article. There’s no need to make any other tricks because WordPress will take care of everything in order to showcase the video in a beautiful way.
You can also have Video Post Format that will give you a field under the editor to input an embed code. To give you an example, this post has these settings in the backend:
If you want Youtube video in a post body, you can embed them the usual way using the embed code.