For customize behavior of search you need know the mechanism of how Magento 2 stock and collect infos and how use it, Now in Magento 2 we have in database catalogsearch_fulltext_scope1 and catalogsearch_fulltext_scope2 , is related to how many scope you have , let's imagine you have 2 store one for English and other for Arabic , the system create 2 scope . And stock the map of search inside this tables , but how ? Yeah let's go and see the structure of table ,this table have 3columns entity_id and atrribute_id and data_index :
entity_id : id of entity or product that mapped with this infos
atrribute_id : id of attribute related to attribute set of product by default title and short description and description attribute used in map
data_index : the work used for map taking from attribute like title or description
the system use the module Catalog_Search for manage creation and re-index of catalogsearch_fulltext_scopeX ,yes for creation the fulltext table Magento 2 use the class vendor/magento/module-catalog-search/Model/Indexer/IndexStructure.php , we have here the method create ,with create we call createFulltextIndex() function , but you need see the variable $this->indexScopeResolver we pass 2 parms for getting the indexer table name by scope or default value the class responsible is: vendor/magento/framework/Indexer/ScopeResolver/IndexScopeResolver.php .
Ok when you create new index command the system will use the class vendor/magento/module-catalog-search/Model/Indexer/Fulltext.php , for update the table in db with new value of column data_index , in this class we have many way and function of indexing (full index or by entity id)
when you create an attribute with option of search-able option (use in search) , the system add automatically this attribute to catalogsearch_fulltext_scope for indexing .
Ok we have other table search_query , that table stock the query text with number of results , popularity active updated_at and other information , the system use for report and sort and give more statistic to administrator , and augment the user experience .