I think under "appear online" the author meant, that when creating a folder in the uploads folder, it doesn't appear in the Media Library.

For this specific solution, I know only one plugin, but since we're not talking about using plugins, maybe you can consider just creating a custom taxonomy "Folders" for custom post type attachments. It is kind of a lot, but can be easily integrated inside your own plugin or WordPress theme, for example, the whole step by step process can be found here https://rudrastyh.com/wordpress/media-library-folders-without-plugin.html