When creating a mobile app, like Instagram, is it created with one language or do developers make it once in Swift and then again in Java?

The simple answer to this question would be that both the scenarios are possible based on the requirement. You can develop an app like Instagram with java for android and Swift for iOS if you want the app to offer native apps experience.

If you want the app to offer a consistent user experience on both platforms then the best suitable option is to use a cross-platform technology like Flutter which uses the same code structure for both platforms.

So developing an app like Instagram completely depends on the offerings you wish to implement within the app.