How to override a service class in Drupal using a service provider class?

In Drupal, you can override a service class using a service provider class. Here are the steps to do so:

1. Create a custom module: Create a custom Drupal module if you haven't already. You can do this by creating a new folder with the module name in the modules/custom directory of your Drupal installation, and adding a *.info.yml file to define the module.

2. Create a service provider class: In your custom module, create a PHP class that implements the ServiceProviderInterface interface. This class will be responsible for overriding the service class. For example, you can create a MyModuleServiceProvider class with the following content:

   <?php
   
   namespace Drupal\my_module;
   
   use Drupal\Core\DependencyInjection\ContainerBuilder;
   use Drupal\Core\DependencyInjection\ServiceProviderBase;
   use Drupal\my_module\CustomLanguageManager;
   
   /**
    * Defines a service provider for the My module module.
    */
   class MyModuleServiceProvider extends ServiceProviderBase {
   
     /**
      * {@inheritdoc}
      */
     public function alter(ContainerBuilder $container) {
       $container->getDefinition('language.language_manager')
         ->setClass(CustomLanguageManager::class);
     }
   
   }
   

   Make sure that your service provide class is named correctly. If the name of your module is my_custom_module, then that modules service provider class name will be MyCustomModuleServiceProvider. More about the naming convention and all that here: https://www.drupal.org/docs/drupal-apis/services-and-dependency-injection/services-and-dependency-injection-in-drupal-8 
   In the above code we are overriding the language.language_manager service.

3. Clear Drupal's cache: After creating the service provider class, you need to clear Drupal's cache to ensure that the new service provider is recognized by Drupal. You can do this by running the following Drush command: `drush cr`

4. Use the custom service: You can now use the custom service in your module or other parts of your Drupal site, and Drupal will automatically use the overridden service with your custom logic.

By following these steps, you can successfully override a service class in Drupal using a service provider class in a custom module.