how I can make the user’s preferred currency, rather than always using US dollars in iExpense app?

Try this out...

Text(item.amount, format: .currency(code: Locale.current.currencyCode ?? "USD"))

This should use the user's default currency or USD if nothing is found.

True, this will change the currency symbol. But the currency formatting (decimal separator, thousands separator, position of the symbol, separation between symbol and number) will retain the current locale, which may not be what is desired. The rules that govern this formatting are complex. Sometimes changing settings on the device (or simulator) will do. In other cases the development language is the determining factor, and this must be set in a configuration file. For more information see https://developer.apple.com/forums/thread/706739