How does watson discovery do retrive and ranking?
how does ranking take place in discovery? does discovery use same technique as RnR or does it use any other external resources, those outside IBM
Not to confuse RnR and Watson Discovery since they are different services. This blog provides some details to the information you are asking; Top 5 reasons to switch from Watson Retrieve and Rank to Watson Discovery. You might also find this helpful, within Discovery you can improve your query results by performing relevancy training; see the links below.
Now RnR is no longer available and it is part of discovery.