Answers for "wordpress get all terms for custom taxonomy"

show all terms of a custom taxonomy

$terms = get_terms( array(
  'taxonomy' => 'taxonomy_name_or_slug',
  'hide_empty' => true,
) );
foreach ($terms as $term){
  echo $term->slug;
  echo $term->name;
  echo "<br><br>";
}

wordpress get terms by taxonomy

You need to pass an additional argument to get_terms(). The default is to hide
"empty" terms-- terms which are assigned to no posts.

$terms = get_terms([
    'taxonomy' => $taxonomy,
    'hide_empty' => false,
]);