ArcGIS Online: Basemap Layer Data Source

[RachaelChasse]

Good Morning,

I am customizing the Light Grey Canvas vector basemap, and using this as my basemap in ArcGIS Pro.  In ArcGIS pro, I need to clip some block group polygons using the light grey LAND area that appears on the basemap.  The problem is, none of the datasets I have found in Portal / Living Atlas match the basemap land boundaries.

Does anyone know what data sources are being used to generate the land area on the default base maps?  Boston has a very detailed coast in this area, with both rivers and oceans contributing--nothing I have tried so far seems to match up enough to make a clean overlay.  The area I am focused on is in the image below. 

Thank you!