As long as the website uses the HTTP/s protocol, usually you can download videos from there.
Yes, they are saved and stored on your computer's hard drive. You can access them by clicking on the toolbar 'Browse' button.
Look for the 'Download' button on the toolbar, clicking it should launch the download.
Unfortunately chrome Web Store does not allow extensions to download videos from Youtube website, but it's enabled for other browsers.
To uninstall video Downloader go to chrome://extensions/ then Remove the extension.