is <audio> and <video> just for local files?
or could I : <video> <iframe> </video> just for semantic reasons...
No, it's not a semantic element, but it's also not for local files. It's for files that can be accessed with a URL (http://www.example.com/myvideo.mp4) The browser does not have access to files on a user's computer unless the file was copied into local storage specifically by the user. I don't see why you would ever need to use an iframe, the <video> tag is much more efficient.
audio and video tags are for server files, not local files. It's only for local file, if the html run from your local machine.
well, you could just use iframe, or if you like youtube, click embed