Where does the query table "taxonomy_term_data_node" come from?
taxonomy_term_data_node is the alias used for the SELECT that is part of the first LEFT JOIN.

LEFT JOIN (
  SELECT td.*, tn.nid AS nid
    FROM {taxonomy_term_data} td                                
    LEFT JOIN {taxonomy_vocabulary} tv ON td.vid = tv.vid
    LEFT JOIN {taxonomy_index} tn ON tn.tid = td.tid                
    WHERE  (tv.machine_name IN ('category_child')
  ) 
) taxonomy_term_data_node ON node.nid = taxonomy_term_data_node.nid