Using various aspects of the WooCommerce API, this can be accomplished using some version of the following.
$order_id = XXX;
$order = wc_get_order( $order_id ); //returns WC_Order if valid order
$items = $order->get_items(); //returns an array of WC_Order_item or a child class (i.e. WC_Order_Item_Product)
foreach( $items as $item ) {
//returns the type (i.e. variable or simple)
$type = $item->get_type();
//the product id, always, no matter what type of product
$product_id = $item->get_product_id();
//a default
$variation_id = false;
//check if this is a variation using is_type
if( $item->is_type('variable') ) {
$variation_id = $item->get_variation_id();
}
//more code
}
Note the documentation,
wc_get_order();
WC_Order();
WC_Order_Item();
WC_Order_Item_Product();