Composer-scaffold is used by requiring drupal/core-composer-scaffold in the project, and providing configuration settings in the extra section of project's composer.json file.

1. Altering Scaffold Files (append/prepend some lines)

Function file_create_url() is deprecated in drupal:9.3.0 and is removed from drupal:10.0.0.

Use file_url_generator service in your service or controller from the container and call its methods.   
 

Before:

$url = file_create_url($uri);

After:

Function drupal_get_path() is deprecated. 

Use Drupal\Core\Extension\ExtensionPathResolver::getPath() in your service or controller. 
 

Before:

If you are using drupal/core-recommended, to update Drupal use this command:

composer require drupal/core-recommended drupal/core-composer-scaffold drupal/core-project-message --update-with-all-dependencies

or

Use the officially supported composer template drupal/recommended-project.

For Drupal 10 version:

composer create-project drupal/recommended-project site_dir

Or also possible to specify the Drupal version: