How to code HTTP Requests?

Question

So I've been learning HTML, CSS, JavaScript and Python for some while.
And I'm at the point where I'm confident enough to create a full-stack project with those skills. However, I'm stuck at the crucial point of trying to connect the backend and frontend.
I'm aware of the REST API, Client Server Model and HTTP Verbs so far.
I want to utilise XML HTTP Requests for JavaScript and either Flask or Request for Python.
(I just want to know how to fundamentally connect the back and front end, and not going for technologies)
Do any of you have any recommendable courses or youtube tutorials? It's been a few days and I'm more or less feeling hopeless :/
Alongside that, if you've a GitHub/Replit/Sololearn portfolio which contains a website with functional communication of the front and back end, I would love to see the source code and see how it works.
I'm just simply stuck and confused... please help me get through this.
Thank you for all.
EDIT:
I'm looking at HTTP requests to:
-> To be able to go to another web page of the web site
-> To be able to send data to the server/backend (Python), for that data to be processed and provide a result for the front end
E.G, lets say I've a calculator website and type in X numbers, I want those numbers to be transmitted back to the Python code, to calculate that, and provide the result to the frontend
(I understand it's easier to do processes like that with JavaScript, however with only 3 months into learning how to program, I wish to know how to communicate and utilise my Python backend code instead)

Answer 1

Answer for this question is a small book. You can search in web and find all what you need. Here an example:
https://www.w3schools.com/js/js_json_http.asp