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

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?',     ],   ]; }