Sorry, my mistake, but actually
trigrams_array is called by trigrams_vector
Definition for the function dict.trigrams_vector:
CREATE OR REPLACE FUNCTION dict.trigrams_vector(word text)
RETURNS tsvector
LANGUAGE sql
IMMUTABLE STRICT
AS $function$
SELECT array_to_string(dict.trigrams_array($1), ' ')::tsvector;
$function$
;