WooCommerce editing PHP to change checkout

Hi,

I'm just learning PHP and am also building a webshop for a client of mine. It is webshop that is supposed to sell liqour. Allthough, the final order and payment won't be happening on the clients website, but on a third party website. So what I am trying to achieve is that the visitor can add products to a cart, and than once done on site1 it will be sent to thirtpartywebsite.com where they will fulfill their order and payment. The link for this is as following:

www.systembolaget.se/Dryckeslista/?a=[ArtikelNr1],[Quantity]&a =[ArtikelNr2],[Quantity]&a =[ArtikelNr3],[Quantity1]

So, I guess I have to wright some PHP code. Something like this:

www.systembolaget.se/Dryckeslista/?a=<?php echo $artikelnr; ?>,<?php echo $quantity; ?>&a =<?php echo $artikelnr; ?>,<?php echo $quantity; ?>&a =<?php echo $artikelnr; ?>,<?php echo $quantity; ?>

And it gets the artikelnr and the quantity from the cart. So somewhere this code has to go, right?

JUST A EXAMPLE (unique for every product)

<?php

$artikelnr = "12345";
$quantity = "1";

?>

This would create this link in HTML

www.systembolaget.se/Dryckeslista/?a=12345,1

My questions are:

Where do I put the above code? Is it in the woo commerce file class-wc-cart.php file?

What is the code to do this automatic for every product in the cart?

How do I get the $artikelnr and $quantity for every product?

I have a feeling this is not way to hard, howevery if you feel that this is pretty advanced coding and that my question is way to much for ask for, please tell me and I will instead hire some outside help for this instead.

With best wishes,

Sebastian