Hi Akshay, if you want to access your static assets that you added under frontend/assets folder then you would have to access like this in the screenshot, check the red box and yellow box to see how I place the image and access the image on browser.

And the reason why you get 405 error is that you don't have default get route handler on your extension.py , but this is not required.

    def get(self, *args, **kwargs):  # pylint: disable=unused-argument
        self.write("Hello World GET: From A extension")

The React example repo is a bit outdated, so we will open a new ticket to make the all dependencies up-to-date.
Let me know if you have any more questions.
Thanks.