Each repository is indexed, but a federator sends out a query to each index – and retrieves the answers.
The developer has two choices
Present the information ranked by data source
Rank and merge the results AFTER the information is retrieved
The first choice is lousy as it requires work by the user.
The second option, which is called a query time merge, requires tremendous compute power, time, and is based on a rules-based ranking mechanism that might be wrong.