Method-1: PHP (print_r / var_dump / var_export)

You may use a combination of the print_r(), var_dump(), var_export(), and always end with die() to break the logic flow, such that the function hault on your debug code and print out:

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  print_r    ("HELLO WIRLD from PRINT_R");
  dump       ("HELLO WORLD from DUMP");
  var_export ("HELLO WORLD from VAR_EXPORT");
  var_dump   ("HELLO WORLD from VAR_DUMP");
  die();

For instance you can have the following code to debug print in custom_theme_name.theme file:

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function custom_theme_name_preprocess_html(&$variables) {
    // Get the current path and node id
    $current_path = \Drupal::service('path.current')->getPath();
    $internal_path = \Drupal::service('path_alias.manager')->getAliasByPath($current_path);
    $website_base_url= \Drupal::request()->getSchemeAndHttpHost();
    $node = \Drupal::routeMatch()->getParameter('node');
    if ($node instanceof \Drupal\node\NodeInterface) {$nid = $node->id();}

    // Debug printing the variables retrived above
    print_r($nid);                   // [DEBUG]: NODE ID
    print_r($current_path);          // [DEBUG]: PATH
    print_r($website_base_url);      // [DEBUG]: WEBSITE BASE URL
    dump($var_array);                // [DEBUG]: HEAD ATTRIBUTE
    die();                           // [DEBUG]: KILL PHP WHEN DONE

    // ===================
    // YOUR CODE CONTINUED
    // ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼
}

Method-2: Drupal (via notification / logger service)

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// Notification pop-out window when visiting the page
\Drupal::messenger()->addMessage("HELLO WORLD from messenger addMessage");
\Drupal::messenger()->addWarning("HELLO WORLD from messenger addWarning");
\Drupal::messenger()->addStatus("HELLO WORLD from messenger addStatus");
\Drupal::messenger()->addError("HELLO WORLD from messenger addError");

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// Logging message visible at "/admin/reports/dblog" after logging in as admin  
\Drupal::logger('logger')->notice("Hello WORLD from logger notice (check log at /admin/reports/dblog) ");
\Drupal::logger('error')->error("Hello WORLD from logger error (check log at /admin/reports/dblog) ");

For instance you can have the following code to debug print in custom_theme_name.theme file:

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// [custom_theme_name.theme] file 
function custom_theme_name_preprocess_html(&$variables) {
    // Get the current path and node id
    $current_path = \Drupal::service('path.current')->getPath();
    $internal_path = \Drupal::service('path_alias.manager')->getAliasByPath($current_path);
    $website_base_url= \Drupal::request()->getSchemeAndHttpHost();
    $node = \Drupal::routeMatch()->getParameter('node');
    if ($node instanceof \Drupal\node\NodeInterface) {$nid = $node->id();}
    
     // Debug printing the variables retrived above
	\Drupal::messenger()->addMessage("hello from node: " . $nid); // Notification  
    \Drupal::logger('logger')->notice("hello from node: " . $nid); // Logging message 
    
    // ===================
    // YOUR CODE CONTINUED
    // ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼
}

For more instruction (e.g. how to use watch dog module to debug), please refer to reference, not recommended as the debug log will eventually be a part of database of website.

Method-3: XDebug (Debug using IDE)

Please refer to reference, very tricky to setup, but can become useful when you are facing complicated scenarios.

Reference