Not sure if "ArcGIS Online" is maps.arcgis.com, so please put it in the correct forum otherwise.
I think I'm misunderstanding something basic. I was told "add data" was not where you actually upload a data source for the experience.arcgis.com builder tool, and I discovered the maps.arcgis.com which has an "add" button and a CSV option, but it appears to be not possible to upload a file and when I serve the file externally paste the URL in the link box it says: "Service error The layer cannot be added to the map."
Any idea how to add data that exists in a CSV file?