How are document-based data stores (e.g. Mongo) and key stores implemented?

Recently, I read a little about document-based databases and key-value repositories (is there a good overview here Is there a difference between document-based databases and key / data-based databases? ), And I had trouble finding good information about the following .

If we request any of them using a key (or an additional index), there is no real difference in the mechanics - get the value. I am unclear how the document repository differs from the value of the key stored when requesting unindexed documents / field. If I had a document store on top of the store’s key value, I would do a “table scan” (check all key / value pairs) for the corresponding value in the query - do document stores do more than this under covers? Is it appropriate to think about data warehouses in documents?

This is a less practical question (I would use Mongo over BDB if I needed to do something useful, most likely) than one aimed at understanding the underlying technology. I am interested in the scaling aspects of specific systems only if they are applicable to the main implementation.

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