Displaying data by views from external API
If all you need to do is display data from the API, then you don't need to do anything special within views.  Just select that you'd like to view rendered entities of your users with a specific view mode. 



In the preprocess function for that view mode, you can hit the API and load the additional data that you need.  Data loaded within a preprocess function should cache, so it shouldn't hit the API on subsequent page loads.

function mymodule_preprocess_user(&amp;$variables) {
  /** @var $account \Drupal\user\UserInterface */
  $account = $variables['elements']['#user'];

  // custom code to hit the 3rd party API