Recent versions of Solr all use BM25 by default. Among other things you see it is using the LuceneQParser (the name of the standard query parser) and, above that, how each result was scored.

The latest milestone in open source development at Bloomberg is the incorporation of the Learning-to-Rank (LTR) plug-in into Apache Solr 6.4.0 ... First, a search query is made for documents ...