The demo is created with a Table/Crosstab/List
item which is a data item with its own DataSource property, different than the report's DataSource.
Details about the structure of the item are available in the related articles in the help section:
The demo report is available in the local examples installed by default under C:\Program Files (x86)\Telerik\Reporting <VERSION>\Examples.
I hope the above information is helpful.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.