need some help with Ai

I want to build a web app that can quickly give a summary of a Reddit user based on their public activity.

The idea is simple: someone enters a Reddit username, the app collects their posts/comments, sends everything to an AI with a predefined prompt, and gets back a summary of what kind of person they seem to be, their interests, patterns, etc.

I've been using Grok for this manually and it does a pretty good job, so I'd like to automate the process. Every time a username is entered, the same prompt should be sent automatically in the background.

The problem is that I only understand the frontend side at a basic level. AI can generate the HTML and CSS for me, but I'm confused about how the backend actually works.

What do I need to learn? APIs? web scraping? Databases? How do I get Reddit data, send it to an AI model, and return the result to the user?

Also, I'm trying to do this as cheaply as possible, ideally for free, so I'd appreciate any suggestions on free tools, services, or open-source alternatives.