php - How to select username in a given query in Joomla?

Helllo,

I have my own Joomla table and I select everything from it to json_encode the result. How can I manipulate some fields in $db->loadObjectList()? I need username from #__users for a given userId.

Here's my code:

<?php
    $query = $db->getQuery(true)
             ->select('*') // fields a and b from this table are userIds
             ->from('#__my_custom_table');
    $db->setQuery($query);
    $result = $db->loadObjectList();

/* how can I select the username from #__users where #__my_custom_table.a and .b is like #__users.id to add these information to $result
I want to override these fields from #__my_custom_table: a, b
*/

  foreach ($result as $row) {
            $row->a = JFactory::getUser($row->a)->name; 
// how to override this in $result?
  }

   echo json_encode($result, JSON_FORCE_OBJECT);
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