The Sirv plugin adds alt tags to images.
WordPress featured images - alt text from WordPress media gallery is used.
Images added from WordPress media gallery - alt text from WordPress media gallery is used.
Images parsed by Sirv plugin (not in WordPress media library) - alt text from the WordPress theme code is used.
'Add Sirv Media' image - alt text from your my.sirv.com account is used. If image had synced from WordPress, any alt text will have been synced also.
WooCommerce product galleries not using Sirv Media Viewer gallery - alt comes from WordPress.
WooCommerce product galleries using Sirv Media Viewer gallery - alt comes from Sirv.