How do I return all terms from multiple taxonomies?

You have all sorts of fun questions today!
There is also a WP_Term_Query available in WordPress. Here is some info on it as well as what parameters you can pass it. https://developer.wordpress.org/reference/classes/WP_Term_Query/__construct/

$term_args = array(
  'taxonomy' => array('tax_one', 'tax_two', 'tax_six'),
  'hide_empty' => false,
  'fields' => 'all',
  'count' => true,
);

$term_query = new WP_Term_Query($term_args);

foreach ( $term_query->terms as $term ) {
  // Here is what lives inside of $term when the 'field' parameter above is set to all
  /*
  WP_Term Object (
      [term_id] => 11
      [name] => General Topics
      [slug] => general-topics
      [term_group] => 0
      [term_taxonomy_id] => 11
      [taxonomy] => category
      How do I return all terms from multiple taxonomies? => 
      [parent] => 0
      [count] => 22
      [filter] => raw
  )
  */
  // Maybe build out your array() in here to NOT have duplicate Terms ??
}

Hope that helps!!