The application_url is the home page of your Shopify admin app.
The redirect_uris are for OAuth.
It’s really up to you how you’d like to handle that.
If I were you, I’d opt to use Shopify’s managed installs and skip trying to implement OAuth by hand. That’s probably most of your problems right now.
Instead, you could set the application_url to be your Shopify Admin app, it handles Shopify session token management, and you can use the Shopify Admin App as a simple client that interfaces to your Django app or directly to your database to store merchant records.