How to get the correct 'base hook' to override a template file in a custom module?

Question

How can I find the 'base hook' to override a template file using a custom module?

'node' and 'user' are pretty straightforward, but others can be a little more elusive.

Example code...

MYMODULE.module:

/**
 ** Implements hook_theme().
 **/
function MYMODULE_theme($existing, $type, $theme, $path) {
  return [
    'username' => [
      'template' => 'username',
      'base hook' => 'how do you get this?',
    ],
  ];
}

Answer 1

Look into the *.module file where the base hook is defined, in this case the User module. The base hook is the key of the template definition:

user.module:

/**
 * Implements hook_theme().
 */

function user_theme() {
  return [
    ... 
    'username' => [
      'variables' => ['account' => NULL, 'attributes' => [], 'link_options' => []],
    ],
  ];
}

Then in your *.module file

mymodule.module

/**
 * Implements hook_theme().
 */
function mymodule_theme() {
  return [
    'username__mymodule_template' => [
      'base hook' => 'username',
    ],
  ];
}

Start the name of your own hook with the base hook and double underscores.