Child theme

function custom_js_css() {
	wp_register_style('customcss', get_theme_file_uri('assets/css/style.css'),
		[], filemtime( get_theme_file_path('assets/css/style.css')));
	wp_enqueue_style('customcss');

	wp_register_script('customjs', get_theme_file_uri('assets/js/script.js'),
		[], filemtime( get_theme_file_path('assets/js/script.js')));
	wp_enqueue_script('customjs');
}

add_action( 'wp_enqueue_scripts', 'custom_js_css', 999 );

Parent theme

function custom_js_css() {
    wp_register_style('customcss', get_template_directory_uri() . '/css/custom.css', [], "1.0");
    wp_enqueue_style( 'customcss');

    wp_register_script( 'scriptjs', get_template_directory_uri() . '/js/script.js', [], "1.0");
    wp_enqueue_script( 'scriptjs' );
}

add_action( 'wp_enqueue_scripts', 'custom_js_css', 999 );

Plugin

    wp_register_style( 'plugin-name-css', plugins_url( 'css/style.css', __FILE__ ), [], filemtime( plugin_dir_path(dirname(__FILE__)) . 'plugin-name/css/style.css') );
    wp_enqueue_style( 'plugin-name-css');