It most likely uses Xamarin, a C# tool for making iOS apps. It gets the job done, and I made a full commercial app for work using it years ago (and I bet it's more solid now). It's kinda cool because most of the Xamarin functions are almost direct wrappers for iOS functions, so you could look something up in Objective-C and 90% of the time know how to do it using Xamarin.

But there are some hiccups with Xamarin, and I'd rather just code native now for iOS only (there's a lot more jobs for native iOS developers than Xamarin, although React Native is a good bet nowadays too). If I was doing cross-platform with Android, I'd either do React Native or I'd consider Xamarin again.