[Plugin: Multiple content blocks] Where are controls

https://plugins.trendwerk.nl/documentation/multiple-content-blocks/

You have to add the trigger to your template file(s) before you’ll see any extra content blocks, e.g. in page.php (or sidebar.php or header.php, etc.), add:

<?php the_block('trigger'); ?>

Then when you edit a page/post that uses that template file you’ll see the additional content/editor labeled ‘trigger’