It uses the search data source which does a keyword search in the episerver search index which has been built from searcable properties. The ranking is (AFAIK) still based on the same algorithm which has been used at least since version 4 of CMS:
Hi, it seems to be a popular confusion and many of you may have answered this question many times.
What does the search function in Alloy Tech sample site use? My current understanding is that it just run the database query to search pages and use Lucene for files. Is it correct?
Also what is the ranking of the search result based on?