Example of creating a plugin instance with the default configuration

CreatePluginInstance.php

<?php

// using PayPal EC as example

/** @var PaymentGatewayManager $plugin */
$plugin = \Drupal::service('plugin.manager.commerce_payment_gateway');

// Get the id of config from `drupal debug:config | grep paypal`
/** @var Config $config */
$config = \Drupal::service('config.factory')
  ->get('commerce_payment.commerce_payment_gateway.paypal_ec');

/** @var ExpressCheckout $ppec */
$instance = $plugin->createInstance('paypal_express_checkout', $config->get());