I see the link you are using points to deploying a Python 2 application in App Engine. Take into consideration that the programming language itself (Python 2) is no longer supported by the community since at least January 1st 2020 so I'd strongly suggest you to use a similar tutorial for Python 3 (find the link here).
In order to serve static files (e.g. an index.html file that you are referring to) App Engine can handle URLs by executing application code (in the specific tutorial you are following this will consist on modifying your Flask application to serve a static file, as e.g. is described here), or by serving static files uploaded with the code and taking advantage of the handler element defined in your app.yaml file.
The comments within the app.yaml file on the tutorial I linked for Python 3 explain this in a very understandable way:
runtime: python39
handlers:
# This configures Google App Engine to serve the files in the app's static
# directory.
- url: /static
static_dir: static
# This handler routes all requests not caught above to your main app. It is
# required when static routes are defined, but can be omitted (along with
# the entire handlers section) when there are no static files defined.
- url: /.*
script: auto
Notice that you basically define a URL and use an element such as static_dir or static_files to serve that file when a customer lands that specific URL in your application.
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