How to work with field collections in $build in hook_entity_view_alter?
HOOK_entity_view_alter() works with render elements. That are all array keys, that have no '#' sign.

At the point you are now there are only fields prepared for rendering on the top level, that means the fields, which are not in a field collection. 

Fields that are inside a field collection have at this point no renderable output.

To get the render elements of these fields you have to use the hook for field_collection_item:

function mymodule_field_collection_item_view_alter(&amp;$build, $entity, $display) {
   kint($build);
}


Here you will be able to unset fields inside the field collection.