commerce-8.x-2.8/modules/tax/src/Resolver/DefaultTaxRateResolver.php

<?php

namespace Drupal\commerce_tax\Resolver;

use Drupal\commerce_order\Entity\OrderItemInterface;
use Drupal\commerce_tax\TaxZone;
use Drupal\profile\Entity\ProfileInterface;

/**
 * Returns the tax zone's default tax rate.
 */
class DefaultTaxRateResolver implements TaxRateResolverInterface {

  /**
   * {@inheritdoc}
   */
  public function resolve(TaxZone $zone, OrderItemInterface $order_item, ProfileInterface $customer_profile) {
    $rates = $zone->getRates();
    // Take the default rate, or fallback to the first rate.
    $resolved_rate = reset($rates);
    foreach ($rates as $rate) {
      if ($rate->isDefault()) {
        $resolved_rate = $rate;
        break;
      }
    }
    return $resolved_rate;
  }

}