How to turn off email you receive when registered?

The function that generates the new user notification is wp_new_user_notification(). It is called by another (similarly named) function wp_new_user_notifications() which is triggered by the action hook register_new_user in the register_new_user() function.

If you follow that (or even if you don’t), all you need to do is remove the wp_new_user_notifications() filter from the register_new_user action:

add_action( 'init', function() {
    remove_action( 'register_new_user', 'wp_send_new_user_notifications' );
});

As others have noted, if you remove this, then depending on how you have the site set up with user passwords, etc, you may have to consider how users will be able to initially access the site.