What folder to place Large Video files?

WordPress doesn’t process uploaded files directly from file system, it stores information about them as attachments in database.

If you need to turn a directly uploaded file into an attachment the term for it would be a sideload. There are some API functions in WP to help with the task, starting with media_handle_sideload().

Also note that none of WordPress is designed to function from private location on web server. Uploads folder is typically public, as is everything in it (knowing URLs to files).