Not sure about your specific cURL script but the base cURL function can be found here inside of WordPress:
https://core.trac.wordpress.org/browser/tags/4.8/src/wp-includes/class-wp-http-curl.php#L220
Also remember modifying WordPress core can cause issues so it’s always best to try to leverage hooks and filters. You may be able to modify the default cURL request via http_api_curl.
https://developer.wordpress.org/reference/hooks/http_api_curl/
This article also breaks down PHP, cURL and WordPress with more detail: https://deliciousbrains.com/php-curl-how-wordpress-makes-http-requests/
Hope this helps!