Why do some sites show themes/”themename” as the only theme?

You can’t remotely inspect site’s files and directories. That’s not what’s happening here. The browser inspector is just reconstructing the structure based on the URLs of resources that it is loading.

The browser sees that /wp-content/themes/themename/style.css is being loaded, and /wp-content/plugins/plugin/style.css and assumes that there must be /wp-content/themes/ and wp-content/plugins/ directories. So it creates a file browser for you based on this structure.

This also means that it is only showing files that the browser is loading. This is not a full (or necessarily accurate) view of the actual file structure on the server.

And the reason that themes/webdesingsun is being shown as a single folder, is simply because the browser is only loading resources from one directory in /themes/, so it’s saving you the trouble of having to expand the only folder in /themes/.