Welcome!
The tags starting with an underscore are non-standard (aka “custom”) tags. Gramps pipes them into a Note so that the data is not lost but otherwise ignores them.
As for the Media object URLs, since they are not locally stored image files, the Gramps thumbnailer plugin does not generate the 2 (normal and large) thumbnails. As you pointed out, instead of an image handling helper application, the URL is linked to a browser as the helper application.
However, there is an addon that will fetch the remotely stored URL image files to local storage : DownloadMedia
Please make a backup and set the relative base path for media in your Preferences before using.