How to pass variables to overridden Twig template located in module?
You need to function MYMODULE_preprocess_node_add_list.

function MYMODULE_preprocess_node_add_list(array &amp;$variables) {
  $variables['hello'] = 'world';
}


This preprocess hook is actually called HOOK_preprocess_HOOK. See the last HOOK in its name? You need to target the 'base hook' which is node_add_list. So preprocess that instead of the page.