List archived posts by subcategory

In your Avada’s child theme update your category.php, if its there or else
create new file with that name and add the code as below:

<?php
$current_cat = get_queried_object();

echo 'Heading: '.$current_cat->name;

$subcategories = get_categories(
    array( 'parent' => $current_cat->term_id )
);
if(!empty($subcategories)){
    foreach($subcategories as $cat){
        echo '<br>'.$cat->name.'<br>';
        $cat_posts = get_posts( array(
            'posts_per_page' => -1,
            'category'       => $cat->term_id
        ) );

        if ( $cat_posts ) {
            foreach ( $cat_posts as $post ) :
                echo $post->post_title.'<br>';
            endforeach;
        }
    }
}else{
    $cat_posts = get_posts( array(
        'posts_per_page' => -1,
        'category'       => $current_cat->term_id
    ) );

    if ( $cat_posts ) {
        foreach ( $cat_posts as $post ) :
            echo $post->post_title.'<br>';
        endforeach;
    }
}

It’ll print exactly as you asked for.

Update:
To get the posts as links, we can add tags like this:

Change everywhere:

echo $post->post_title.'<br>';

To:

echo '<a href="'.get_permalink().'">'.$post->post_title.'</a><br>';

It’ll make posts as links.