Magento 2 : How to run custom script on magento root?

Create a file on magento root and use object manager for using dependencies,

In below example you can see how we get product collection of a particular category using root script.

<?php

//use Magento\Framework\App\Bootstrap;
require __DIR__ . '/app/bootstrap.php';
$bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
$obj = $bootstrap->getObjectManager();

// Set the state (not sure if this is necessary)
$state = $obj->get('Magento\Framework\App\State');
$state->setAreaCode('frontend');
$objectManager = \Magento\Framework\App\ObjectManager::getInstance(); // Instance of object manager

/* category id*/
$cateid = 'CATEGORY_ID';

/* Dependency for loading category products */
$cateinstance = $objectManager->create('Magento\Catalog\Model\CategoryFactory');

$categoryproduct = $cateinstance->create()->load($cateid)->getProductCollection()->addAttributeToSelect('*');
$blankOptionsProductIds = [];
foreach ($categoryproduct as $_product) {
	echo $_product->getId()."<br>";
}

After creating the file run the script on browser or using cli

NOTE : This is not a best practice to build any feature in Magento, this is only for debugging purpose or one time code execution.