Get rendered content of one block created as derivative block
It sems this will do the trick:

  $blockManager = \Drupal::service('plugin.manager.block');
  //see all available block plugins (even not instantiated)
  $contextRepository = \Drupal::service('context.repository');
  $definitions = $blockManager-&gt;getDefinitionsForContexts($contextRepository-&gt;getAvailableContexts());


Here is how I used this info:

function TEMPLATE_preprocess_node(&amp;$vars) {
  $blockManager = \Drupal::service('plugin.manager.block');   
  $vars['content']['derivative_block'] = $blockManager-&gt;createInstance('module_name:block_derivative_id')-&gt;build();
}


If you enable the commented lines $definitions will get you all available blocks 

Credit goes out to enzo for this solution.

More info about programmatic block placement.