IOS applications can be natively written in objective-c, or they can be web applications displaying a mobile service hosted on a site in a WebKit view.
No matter what format Instagram uses for its iOS application, django can still be a solution. As a native application, they can still communicate with the mobile device on the django web server through the REST api interface.
Django should not be used solely for serving HTML. It can serve any HTTP request you want. This is very common for sites when they have an interface for desktop browsers, and then they also expose some API URLs for the same backend, which allows mobile applications to make the same requests.
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