I guess once I download a video it is permanent, right?
Yes.
Maybe it is a silly question but I did download videos using "Save as -->archive" and of course when the video on the site was deleted, I had nothing.
That saved the web page, but that doesn't contain the actual video. That is sent by the server when needed.
I guess it's the same with video download ?
Another reason I ask is if it is so simple to just use Quick Time and it's free and it copy even without asking (all I have to do is watch) why so many ads for "capture videos ?"
Web sites often disable QuickTime's Save function, so you often need other ways to save video.