How to write a server part in Express (node ​​js)?

Question

JavaScript

I have a multi-page layout site and I need to make it possible to take data from the database and display them in cards on the site (like a pizzeria), as well as the Admin panel, for the ability to add products from the site if the Admin and the question is how to do it on express?
In itself, this is not difficult, but I already have a full-fledged layout site and, like, how to attach Express to it. Tipo specify all html, css files so that everything works and looks the same or what? Or is it better to score and do it in PHP somehow?