How taxonomy index work

taxonomy_index.php

<?php
/**
 * Implements hook_ENTITY_TYPE_insert() for node entities.
 */
function taxonomy_node_insert(EntityInterface $node) {
  // Add taxonomy index entries for the node.
  taxonomy_build_node_index($node);
}
/**
 * Implements hook_ENTITY_TYPE_update() for node entities.
 */
function taxonomy_node_update(EntityInterface $node) {
  // If we're not dealing with the default revision of the node, do not make any
  // change to the taxonomy index.
  if (!$node->isDefaultRevision()) {
    return;
  }
  taxonomy_delete_node_index($node);
  taxonomy_build_node_index($node);
}