I am doing Apple Pay web integration in my payment gateway using applepayjs so merchants can use Apple Pay but the entire code including Apple Pay button is hosted in payment gateway domain so merchants has to create merchant id and cert in this case

I am doing apple pay web integration using apple pay.js for a payment gateway and the complete Apple Pay web integration code is deployed with payment gateway domain and merchant can leverage the Apple Pay web using an iframe in their website. So in this case each merchant use this Apple Pay web register a merchant Identifier and hence merchant certificate also if we need to handle different cert for different merchants? How this work? I couldn’t see any documentation for payment gateway Apple Pay web integration please advise