Q: what is %d used for in arrays on wordpress? I presume it means double but what does it mean in general?
 $wpdb->delete(
      "data",
      [ 'id' => $id ],
      [ '%d' ]
    );

I get that I'm deleting from table data using array id => id but what is the %d for?  
Why am I asking?  well, I'd like to do some bulk updates as shown through the features built in with wp_list_table as shown through this tutorial.  As I've looked around for that first link I found a few more sources on how to fix my table - I'm just trying do multiple operations and I'm stuck with bulkupdater

A: its to identify the type of data to be deleted think where=%d which means interger
others include:
%d - interger (just to make it clear)
%f - float
%s - string

so your code 
$wpdb->delete(
  "data",
  [ 'id' => $id ],
  [ '%d' ]
);

Is only going to delete an id where the id string is an integer e.g. 43 not if a43
You can have obviously a few types in an array, but they need to match the order of the data array (i.e. array('id'=>$id, 'numval'=>$num) ,array(integer, integer) )
