List Categories and Taxonomies in Template.

All regular Post Categories

get_categories( string|array $args = '' );

//returns Array of category objects. See get_terms() for possible $args for filtering returned category objects. 

wp_list_categories(array|string $args = '');
//output categories as linked HTML to each category archive page. Can pass argument of echo=true to output directly.

All Terms in Taxonomy

get_terms( array(
    'taxonomy' => 'post_tag',
    'hide_empty' => false,
) );
//returns Array of category objects. See below for possible $args.
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