Create widgets in wordpress by codex.

 add_filter( 'use_widgets_block_editor', '__return_false' );

// Register sidebars by running tutsplus_widgets_init() on the widgets_init hook.

add_action( 'widgets_init', 'tutsplus_widgets_init' );

function tutsplus_widgets_init() {

    // First footer widget area, located in the footer. Empty by default.

register_sidebar( array(

'name' => __( 'First Footer Widget Area', 'tutsplus' ),

'id' => 'first-footer-widget-area',

'description' => __( 'The first footer widget area', 'tutsplus' ),

'before_widget' => '<div id="%1$s" class="widget-container %2$s">',

'after_widget' => '</div>',

'before_title' => '<h3 class="widget-title">',

'after_title' => '</h3>',

) );

}

Overrite wordpress template file using hooks.

    function my_plugin_custom( $template ) {

if( is_404() ){

        $template =  get_template_directory(). '/templates/404.php';

    }

return $template;

}

add_filter( 'template_include', 'my_plugin_custom' ); 

Error page redirect on 404 page.

 add_action( 'template_redirect', 'unlisted_jobs_redirect' );

function unlisted_jobs_redirect() {

if(is_404()) {

global $wp;

$link = home_url($wp->request);

$link_array = explode('/',$link);

$page_not_found = end($link_array);

if($page_not_found != 404) {

wp_redirect( home_url( '/404/' ) );

exit();

}

}

}        

Add custom text on product page.

 add_filter( 'woocommerce_cart_item_name', 'custom_text_cart_item_name', 10, 3 );

function custom_text_cart_item_name( $item_name, $cart_item, $cart_item_key ) {

        $item_name .= '<br /><div class="item-shipping-class">'Here is add some text'</div>';

    return $item_name;

}