how-to-use-magento2-function-in-root-file.php

<?php

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

use Magento\Framework\App\Bootstrap;
require __DIR__ . '/app/bootstrap.php';

$bootstrap = Bootstrap::create(BP, $_SERVER);
$objectManager = $bootstrap->getObjectManager();
$state = $objectManager->get('\Magento\Framework\App\State');
$state->setAreaCode('admin'); //frontend

EX :-
$feed = $objectManager->get('FILE PATH');
$feed->METHOD NAME();