NASA (https://www.nasa.gov/open/nebula.html) uses Solr for its Nebula Cloud Computing Platform. Similarly, Public Library of Science (PLOS) that is a non-profit publisher of research articles on various subjects. VUFind (https://vufind.org/vufind/) is another powerful open source discovery portal of libraries. It is known to have around 25 million records for a few of its implementations.