SharePoint Admin Questions, for SharePoint Online.

/sites is basically just a site collection path I forget the technical term of it but your sites under it are site collections not subsites. The sites your creating in sites can be viewed in the new SharePoint admin center which you can click in top right corner this will have your active sites which are all the /sites and other modern site collections etc.

The existing root URL is just a root site collection that is listed because it’s a classic site collection as any other old school site collections might show in the old admin center.