How/where does Flask-Caching store data?

I just wonder how and where the response is stored when using Flask-Caching.

For example:

from flask import Flask, request
from flask_caching import Cache
import datetime

app = Flask(__name__)
cache = Cache(app, config={'CACHE_TYPE': 'simple'})

def make_cache_key(*args, **kwargs):
    return request.url

@app.route('/', methods=['GET'])
@cache.cached(timeout=50, key_prefix=make_cache_key)
def foo():
    time = str(datetime.datetime.now()) + " " + str(request.url)
    return time, 200

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