BlockContent::load for addtoany_block returns null
The addtoany_block is not a block content, but it's a plugin block.
So if you want to get the its build programmatically you should:
1. Get the block configuration (addtoany_block settings).
2. Create an instance of the block.
3. Get the build.

Try the following:   

// Block manager.
$block_manager = \Drupal::service('plugin.manager.block');
// Load Block settings / Settings.
$addtoany_config = \Drupal::config('addtoany.settings')-&gt;getRawData();
/** @var \Drupal\addtoany\Plugin\Block\AddToAnyBlock $plugin_block */
$addtoany_block = $block_manager-&gt;createInstance('addtoany_block', $addtoany_config);
// Get Block build.
$build = $addtoany_block-&gt;build();