Access to Feature Layer is provided in the following ways:
View it online in Map Viewer, which you can do without a login or Arc license.
Open the layer in ArcGIS Desktop, which obviously requires a product license. I assume this connects to the layer through the web feature service rather than downloading the source data.
There's also a URL, which from my limited understanding I believe is a Web Feature Service URL, eg
(I've not succeeded at adding any layer to QGIS from one of these service URLs, either as a WFS or ArcGISFeatureServer layer, but that's a separate question.)
Is it possible to download an online ArcGIS "Feature Layer" in any geospatial vector format (shapefile, geojson, geopackage, CSV with WKT or other-formatted geometry column, etc.) with open-source software? Or are they available online only?
I found a Geoprocessing Tool called Download ArcGIS Online Feature or AGS Feature/Map Service, but it appears to be something you need ArcGIS to run. But the existence of that tool implies that it's possible...