Displaying one blog category on its own page with subcategories

// Get the Humor category
$cat = get_term_by( 'slug', 'humor', 'category' );

// Get the sub-categories under Humor
$sub_cats = get_terms(
    'category',
    array(
        'child_of' => $cat->term_id,
        'hide_empty' => false,
    )
);

// Collate the term ids of all the sub-categories AND the parent category
$cat_ids = array_map(
    function ( $value ) { return $value->term_id; },
    $sub_cats
);
array_unshift( $cat_ids, $cat->term_id );

// Retrieve only posts belonging to Humor and its sub-categories
$cat_posts = get_posts( array( 'category__in' => $cat_ids ) );
foreach ( $cat_posts as $cat_post ) {
    var_dump( $cat_post->post_title );
}

Strictly speaking, “Humor” would be a term under the “Categories” taxonomy (Admin > Posts > Categories). For example, I can create a new “Colors” taxonomy, add “Red”, “Green”, “Blue”, and assign them when editing a post. Just FYI 🙂

For Question #1:
Using the example of the Humor category, you can put the code in page-humor.php, where “humor” is the the slug of the Humor page (presumably set up to show all the posts from the Humor category). Reference: https://developer.wordpress.org/themes/template-files-section/page-template-files/page-templates/#page-templates-within-the-template-hierarchy

For Question #2:
Probably Dashboard > Categories?

For Question #3:
The sorting can be done via the orderby and order arguments in the get_posts() call. Reference: https://codex.wordpress.org/Template_Tags/get_posts

It appears that the text from the page IS showing up. Sorry for that mistake. But the categories are not showing up.