Essentially the whole concept of LNURL is that the seller (e.g. the Restaurant) has to run a website (e.g. restaurant.com) that knows how to respond to a request with an LN invoice. So when you scan that static LNURL QR code with your mobile phone, the QR code just has the full link inside of it, e.g. restaurant.com/getinvoice/some_ID_123 and it returns "classic" LN invoice. Your mobile wallet know that when it reads QR code that has a link in it, it's supposed to make a request to the restaurant.com website to get invoice and pay it.