Hi @JayLarsen,
If I understand correctly, you want to enrich a layer with demographic data. To get started in Map Viewer,
Click Analysis,
Search the analysis tools for Enrich,
and select the layer (Cities) you want to enrich.
Then you can select the demographic info you want.
Check out the ArcGIS Online Help documentation on for information on the Enrich layer tool https://doc.arcgis.com/en/arcgis-online/analyze/enrich-layer-mv.htm
You'll need the proper permissions and credits to use the Analysis tools.
Hope that helps!