Problem on creating custom endpoints for REST

I am having trouble on creating custom endpoints to extend my Wordpress application.

After setting up the WordPress module, I can access the son data through the link given :

http://localhost/wordpress/wp-json/

I tested different endpoints inside the document with link:

https://developer.wordpress.org/rest-api/reference/

And now I am trying to create my own endpoints, but after many researched I could only find something like

add_action( 'rest_api_init', 'myplugin_register_routes' );

and then

function myplugin_register_routes() {
  register_rest_route( 'myplugin/v1', 'foo', array(
    'methods'  => WP_REST_Server::READABLE,
     'callback' => 'myplugin_serve_route',
  ));
}


function myplugin_serve_route( WP_REST_Request $request ) {
    // Do something with the $request
    // Return either a WP_REST_Response or WP_Error object
    return $response;
}

But indeed where should I add these things? Also, I researched a lots and see the advanced endpoint controller practice, may anyone give a hand on me? Or I need to create my own plugin?