Where are the layers added from files to Map Viewer stored in Portal for ArcGIS?

Answer

In Map Viewer, layers can be added from files to be displayed on the map. However, these layers differ from the layers added through other methods in terms of how the layers are stored in Portal for ArcGIS. Layers added as a hosted feature layer or an ArcGIS Server layer are stored on ArcGIS Data Store as an independent item and the item details page is accessible when clicking the feature layer in the Portal for ArcGIS Contents page.

Layers added to the map from files are instead saved as part of the web map itself in the web map’s JSON definition file. To view layers stored in a web map’s JSON definition file, follow these steps using ArcGIS Online Assistant:

Warning:   
ArcGIS Online Assistant is not an official product of Esri. Esri cannot guarantee accurate results following incorrect use of the product.

1. Navigate to ArcGIS Online Assistant, and select Log in to Portal For ArcGIS.
2. On the Menu bar, click I want to... > View an Item's JSON.
3. From the Root drop-down menu, navigate to the web map, and click Item ID.
4. Navigate to the Data section of the web map's JSON definition. All layers in the web map, including the layers added from files are stored in this section.

For a layer added from a file to have its item details page accessible, the layer must be saved as its own item. Refer to Portal for ArcGIS: Save layers for more information on saving the layer.