Can I use Flutter for backend?

You won’t be able to convert python django application into a native mobile application. For web application, both the client side and server side code is rendered through web browser.

A point to remember here is that in order to work with Angular, on the Django part we’ll create a home URL as defined below on line 9 and the other URLs will be the REST API which the Angular part will use to connect to Django.

Yes . Use the standard Android app tools and use Django to serve and process data through API requests.