Given a example directory structure that looks like this
app/static/css/bootstrap.min.css
app/static/js/bootstrap.min.js
app/templates/test_page.html
app/views
run_server
You can set the flask static_url_path when you create the app object
app = Flask(__name__, static_url_path='/static')
and then in test_page.html you can have something that looks like this
<link href="{{ url_for('static', filename='css/bootstrap.min.css') }}" rel="stylesheet">
I am an HTML body
<script src="{{ url_for('static', filename='js/bootstrap.min.js') }}"></script>
While it's not required to set the static URL path, since what's shown above is the default, I want to show it because it's a feature I didn't know about for a long time and can be useful.