Elasticsearch

Magento 2.3 still use "OR" logic for mysql and elasticsearch. Plus "should" conditions (query to ES) are wrong. That's why relevance almost same. But ES significantly improve performance of search and catalog (layered navigation).

you have to set AND operator in your elastic search json queries, Override the module-elasticsearch/SearchAdapter/Query/Builder/Match.php. file in your custom module and then The code you have to edit in the build function in line no 75

You can use the Magento 2 CLI to do it, you can check your current search engine using:

php -f bin/magento config:show catalog/search/engine

if you want to change the engine to Elasticseach version 6 then use this command 

php -f bin/magento config:set catalog/search/engine 'elasticsearch6'

We can disable Disable Smile-SA / Elasticsuite in Magento by executing this command 

php bin/magento modeul:disable Smile_ElasticsuiteCore Smile_ElasticsuiteCatalog Smile_ElasticsuiteCatalogGraphQl Smile_ElasticsuiteCatalogRule Smile_ElasticsuiteCatalogOptimizer Smile_ElasticsuiteTracker Smile_ElasticsuiteThesaurus Smile_ElasticsuiteSwatches Smile_ElasticsuiteIndices Smile_ElasticsuiteVirtualCategory

to install the needed plugins you need to execute this command 

To display the status of the Elasticseach please use this command

curl -X GET 'localhost:9200/_cat/health?v&pretty'

to display the current installed Elasticsearch plugin use this command 

curl -X GET "localhost:9200/_cat/plugins?v&s=component&h=name,component,version,description&pretty"