How do I set the default value for results?
Initially I tried some different approach:

extending SearchApiString
an event subscriber of KernelEvents::REQUEST altering the incoming url

because by default facets\search api rely on the query_params of the url and the &quot;values&quot; of the relative facets various objects of the modules.
Both of those those approaches kinda failed, because the first didn't update the query params of the request, while the second caused some bugs that I couldn't manage to solve.
The working approach was to extending the default url_processor QueryString - which both initialises the active facets filters and build the url after a search.
Injecting the logic when the class is created allowed to do a minor modification without breaking all the standard functionality.
&lt;?php

namespace Drupal\my_module\Plugin\facets\url_processor;

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\facets\Plugin\facets\url_processor\QueryString;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Query string URL processor.
 *
 * @FacetsUrlProcessor(
 *   id = &quot;my_query_string&quot;,
 *   label = @Translation(&quot;My Query string&quot;),
 *   description = @Translation(&quot;Add description&quot;)
 * )
 */
class MyQueryString extends QueryString {

  /**
   * A string of how to represent the facet in the url.
   *
   * @var string
   */
  protected $urlAlias;

  /**
   * The event dispatcher.
   *
   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
   */
  protected $eventDispatcher;

  /**
   * {@inheritdoc}
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, Request $request, EntityTypeManagerInterface $entity_type_manager, EventDispatcherInterface $eventDispatcher) {
    //this is where I'm adding a default value
    $this-&gt;alterRequest($request);
    parent::__construct($configuration, $plugin_id, $plugin_definition, $request, $entity_type_manager, $eventDispatcher);
  }

  /**
   * @param \Symfony\Component\HttpFoundation\Request $request
   */
  private function alterRequest(Request $request): void {
    $query = $request-&gt;query;
    $facet_new_parameters = [];

    if ($facet_parameters = $query-&gt;get('f')) {
      $content_type_filter_exists = FALSE;

      foreach ($facet_parameters as $parameter) {
        $sepator = ':';
        $explosion = explode($sepator, $parameter);
        $facet_type = array_shift($explosion);
        if ($facet_type === 'content_type') {
          $content_type_filter_exists = TRUE;
        }
      }

      if (!$content_type_filter_exists) {
        $facet_new_parameters = array_merge($facet_parameters, ['content_type:prodotto']);
      }
    }
    else {
      $facet_new_parameters = ['content_type:prodotto'];
    }

    if (!empty($facet_new_parameters)) {
      $query-&gt;set('f', $facet_new_parameters);
    }
  }
}