How to work with a file in Python that user input into HTML

I am creating a webapp using React that takes a csv file as input from user. I want to then work with that csv file in python, then return it to the user so that they can download it.

This is what I have so far:

React (Frontend):

App.js:

function App() {
    return(
        <UploadFile />
    );
}

UploadFile.js:

function Upload(){
return(
    <form action="/action_page.php" method="POST" encType="multipart/form-data">
        <input type="file" id="dat" name="filename"/>
        <input type="submit"/>
    </form>
);
}

Python (Backend): working.py

from flask import Flask, render_template, request
from werkzeug import secure_filename

app = Flask(__name__)

@app.route('./upload')

def upload_file():
    return render_template('index.html')

@app.route('/uploader', methods = ['POST'])
def upload_file():
    f = request.files['file']
    f.save(secure_filename(f.filename))
    return 'File uploaded'

if __name__ == '__main__':
    app.run(debug = True)

I'm completely lost on how to continue. I've sort of followed ideas from Taking user input from HTML form as a variable for Python script and https://pythonbasics.org/flask-upload-file/ but I'm not sure how to continue.

Where is the file saved and what is it saved as? How do I start using it? And is the secure_filename function required?

Can anyone help me on the right track to start using the actual file?

Thanks