get all parent categories, and then based on parent id you can write a recurring function that gives you the child category of given id.

function wpdocs_get_child_categories( $parent_category_id ) {
    $html = '';
    $child_categories = get_categories( array( 'parent' => $parent_category_id, 'hide_empty' => false, 'taxonomy' => $taxonomy ) );
    if ( ! empty( $child_categories ) ) {
        $html .= '<ul>';
        foreach ( $child_categories as $child_category ) {
            $html .= '<li>'.$child_category->name;
            $html .= get_child_categories( $child_category->term_id );
            $html .= '</li>';
        }
        $html .= '</ul>';
    }
    return $html;
}

function wpdocs_list_categories() {
    $html = '';
    $parent_categories = get_categories( array( 'parent' => 0, 'hide_empty' => false, 'taxonomy' => $taxonomy ) );
    $html .= '<ul>';
    foreach ( $parent_categories as $parent_category ) {
        $html .= '<li>';
        $html .= $parent_category->name;
        $html .= wpdocs_get_child_categories( $parent_category->term_id  );
        $html .= '</li>';
    }
    $html.= '</ul>';
    return $html;
}
add_shortcode( 'wpdocs_list_categories', 'wpdocs_list_categories' );

Output look like this:

A – Main category
B    – Child Category
C    – Child Category
D      – Nested Child Category( if available )
E – Main category
F    – Child Category
G    – Child Category
H     – Nested Child Category( if available )