How can I connect a front-end code, hosted via Firebase, with a back-end code?

I am developping a website using Javscript, HTML and CSS, and which is hosted via Firebase.
However, I would like to implement backend code on my server, as it would be easier to perform a search through the huge (135MB) JSON file I have.

If I code a Node.js or a Python file that would do this for me, how can I connect it to my website or to Firebase ?

I heard that I can use socket.io, or create an API to do so, but I have no idea how to implement it.