I want to load a plugin definition by its string id within a controller extending ControllerBase. What is the relevant service or class I am looking for?
You can use the plugin manager to fetch a plugin. You only need to find the correct plugin manager service:
$type = \Drupal::service('plugin.manager.service'); // Fill your own plugin manager service. $plugin_definition = $type->getDefinition('plugin_id'); // Fill needed plugin id.