This whitepaper from the ArcGIS Enterprise product management team, discusses the two ways data can be handled with the ArcGIS Platform:
User Managed: where data owners and admins are responsible for providing and managing the underlying storage infrastructure, e.g., file servers and database servers.
ArcGIS Managed: the concept of hosted data because the data is stored within (hosted by) the ArcGIS system and exposed solely through REST based web services that are shown to end users as layer items within their portal.
Also for ArcGIS Enterprise users, the ArcGIS Data Store is not a replacement for existing enterprise geodatabases.
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Enjoy,
Derek, thank you for explicitly pointing out "ArcGIS Data Store is not a replacement for existing enterprise geodatabases." It isn't uncommon for casual/typical GIS users to read Esri's marketing and documentation and jump to overly simplistic conclusions about the data management landscape with Esri products.
I haven't read through the document yet, although I intend to soon, but another area of misconception is "managed data store." I think your descriptions above are good, when looking at it from the user's perspective, but it is common for users and management types to think "ArcGIS Managed" means you don't need anyone managing them because they are "ArcGIS Managed." From the user perspective, the ArcGIS Data Stores may be "ArcGIS Managed," but from an IT service providers perspective there is still plenty of work involved in managing them.