The web service url ends with "MapServer" will be valid url, i.e., https://server-domain/arcgis/rest/services/service-name/MapServer
A layer in a web service can be valid url, i.e., https://server-domain/arcgis/rest/services/service-name/MapServer/0. Click on each symbol for attribute information.
A layer control of added layer is created in the Reference layer list. You can turn on/off and change the transparency.