Where does a theme hook come from?
The hook page comes from a hook_theme implementation in system.module:

/**
 * Implements hook_theme().
 */
function system_theme() {
  return array_merge(drupal_common_theme(), [
    ...


which calls the function drupal_common_theme() in theme.inc:

function drupal_common_theme() {
  return [
    // From theme.inc.
    'html' =&gt; [
      'render element' =&gt; 'html',
    ],
    'page' =&gt; [
      'render element' =&gt; 'page',
    ],
    ...


The hook name is the top key of the array in hook_theme. See the documentation in D8 Theme API. The theme suggestions are built in core the same way as in custom code.