class Walker_Category extends Walker {
    public $tree_type = 'category';
    public $db_fields = [
        'parent' => 'parent',
        'id'     => 'term_id',
    ];
    
    public function start_lvl( &$output, $depth = 0, $args = [] );
    public function end_lvl( &$output, $depth = 0, $args = [] );
    public function start_el( &$output, $data_object, $depth = 0, $args = [], $current_object_id = 0 );
    public function end_el( &$output, $data_object, $depth = 0, $args = [] );
}

public $db_fields = [
    'parent' => 'parent',    // WP_Term->parent
    'id'     => 'term_id',   // WP_Term->term_id
];

public function start_lvl( &$output, $depth = 0, $args = [] )

<ul class='children'>

public function end_lvl( &$output, $depth = 0, $args = [] )

</ul>

public function start_el( &$output, $data_object, $depth = 0, $args = [], $current_object_id = 0 )

<li class="cat-item cat-item-5 current-cat">
    <a href="/category/news/" aria-current="page">News</a> (42)

<a href="/category/news/">News</a> |

<a href="/category/news/">News</a><br />

public function end_el( &$output, $data_object, $depth = 0, $args = [] )

</li>

// Direct usage
$walker = new Walker_Category();
$categories = get_terms( [
    'taxonomy'   => 'category',
    'hide_empty' => true,
] );

echo '<ul>';
echo $walker->walk( $categories, 0, [
    'style'             => 'list',
    'show_count'        => true,
    'use_desc_for_title' => true,
    'current_category'  => get_queried_object_id(),
] );
echo '</ul>';

// Via wp_list_categories() with custom walker
wp_list_categories( [
    'walker' => new My_Custom_Category_Walker(),
    'title_li' => '',
] );

class My_Category_Walker extends Walker_Category {
    public function start_el( &$output, $category, $depth = 0, $args = [], $current = 0 ) {
        // Add icon before category name
        $icon = get_term_meta( $category->term_id, 'icon', true );
        $output .= '<li>';
        $output .= '<a href="' . esc_url( get_term_link( $category ) ) . '">';
        if ( $icon ) {
            $output .= '<span class="icon">' . esc_html( $icon ) . '</span> ';
        }
        $output .= esc_html( $category->name );
        $output .= '</a>';
    }
}