Customer vat number variable

Hi,
I want to print my customer vat number on invoices.
I understood your guide (https://docs.wpovernight.com/woocommerce-pdf-invoices-packing-slips/displaying-a-custom-field/) about custom code in functions.php, but I can’t find my vat number variable.
Could you please help me find that without plugins?
In which php file do I find it?
Thanks