I am confused in interpreting the below definition:
An extension schema does not have its own schema: the schema generated from the source schema will be filled in with the fields of the extension schema. What I understand is the source schema will be filled in with the updated fields in extension schema. Is this correct?
Extension schema is just a view of source schema in which new columns will be added. Out of the box schemas in adobe campaign cannot be modified directly, but they can be extended and modified. All the columns added in the extension schema will be reflected in the source schema as they point to one schema in the database.