How to display txt file contents in a HTML page?

First, to clarify, this question may have a lot of misunderstandings of how HTML works, sorry, it's my first time and I'm working with a pre-built program I've been provided with.

So I have a python based program, App.py, and I also have index.html

@app.route('/')
def home():
    return render_template('index.html', server_url=BUGGY_RACE_SERVER_URL, Line=line)

when app is run, it's routing over to index.html.
Now I also have a txt file, which the contents of need to be displayed by index.html. On the Python side, I know how all the programming works, I can read the text, format it however I need, I just do not know how to pass that in or store it in such a way that I will be able to have index.html display it.

I'll put below the full code of both app.py and index.html in case they're needed. Sorry if I've missed anything important to explain or have not properly explained the issue, I'll answer any questions.

app.py -

from flask import Flask, render_template, request, jsonify
import os
import sqlite3 as sql


app = Flask(__name__)


DATABASE_FILE = "database.db"
DEFAULT_BUGGY_ID = "1"
BUGGY_RACE_SERVER_URL = "https://rhul.buggyrace.net"


file1 = open("costs.txt")
line = file1.readline()


#------------------------------------------------------------
# the index page
#------------------------------------------------------------

@app.route('/')
def home():
    return render_template('index.html', server_url=BUGGY_RACE_SERVER_URL, Line=line)

index.html -

<h1>
    My buggy editor
  </h1>

  <div class="spacer">
    <a href="/new" class="button">Make buggy</a>
    <a href="/buggy" class="button">Show buggy</a>
    <a href="/json" class="button">Get buggy JSON</a>
  </div>



  <p>
    

    Use this editor to specify a racing buggy. The editor saves it in its
    little database and generates JSON data for the buggy. This is the data you
    need to supply when you log into the
    <strong><a href="{{ server_url }}">race server</a></strong>
    and enter your buggy into the next race.
  </p>
  <p>
    Remember that if your data is not accepted by the race server, your buggy
    will be disqualified from that race... so make sure you program your editor
    correctly.
  </p>

{% endblock %}

You are almost there. Look at this line

return render_template('index.html', server_url=BUGGY_RACE_SERVER_URL, Line=line)

Here, you are rendering your index.html template and pass some content to it, server_url being one such content (and that's already displayed in your HTML) and Line being another.

So, you will need to put

{{ Line }}

to the place in your index.html where you want this content to be displayed at. It should display the line you have loaded from the file because of your

file1 = open("costs.txt")
line = file1.readline()