Crafter Search is the search facade for Crafter CMS. Crafter Search proxies the real search engine deployed behind it.
Crafter Search and Solr is supported for backward compatibility. Crafter CMS’s default search engine is Elasticsearch.
To update your sites to Elasticsearch, follow the guide Migrating a site from Solr to Elasticsearch
Crafter Search’s source code is managed in GitHub: https://github.com/craftercms/search
Crafter Search’s Java Doc is here: http://javadoc.craftercms.org/3.1.16/search/index.html