How to bind a backend to a Pug + Gulp project?

Question

Node.js

Hello! There is a project that is written using the Pug template engine, split into separate files in accordance with BEM, the SASS preprocessor is used and all that. In general, everything seems to be done according to the mind. All this is collected into static .html files using the Gulp assembler.

The question arose - how to add a backend to all this? To be honest, I don't quite understand. I used to change the page extension from .html to .php and just write PHP code right there. And how now correctly to add it? So that you can, for example, request data from other resources, store it in a database, process it in some way, and so on. If there is such an opportunity, I would like to make the server part on NodeJS, since I have already got used to JS in the browser part, but I just want to try it. Thanks in advance!