Full text search is the core App of a full-text search framework for your Nextcloud.
To have it operate, and get content indexed, some other Apps are needed:
Some Providers Apps to extract content from your Nextcloud.
A Platform App that communicate with a search platform (ie. Elastic Search, Solr, ...) in order to index the content provided by the Providers. Note: There is no limit to the number of Platform-App to be installed, however only one can be selected from the Admin Interface