Hey @razvan!
Great question since no one wants to get in trouble for copyright infringement. Here is a Community Guide that shows how YouTube videos can be added: How do I link to or embed a YouTube video in the Rich Content Editor? . YouTube videos are added by many instructors for various reasons and are fine to add. If the author of the video does not want their content embedded in another site, there is a box that they can check when they are publishing the video on YouTube.
Hope this helps!
