@LaurenBoyd Thank you.
We do have a ArcGIS Portal & Server (10X) setup, which I believe you are referring to as ArcGIS Enterprise. The reason behind using ESRI Base maps directly inside our app (ArcGIS Javascript API 4X based) was so we can use external tiles directly into app instead of proxying it through Portal.
So in our use case, do we still need a Key to use ESRI hosted Base map?
Thanks again for the response and licensing terms.