Elasticsearch is a search engine based on the Lucene library. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents.
You can find their documentation at https://www.elastic.co/guide/index.html

MongoDB is a cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with schema. You can connect to a mongodb server using robo3t, or the mongodb command line.
You can find their documentation https://docs.mongodb.com/