You can have a team with iOS and Android developers to your apps, that is more expensive or you can use frameworks where you write once and compile to the desired platform.
To this kind the frameworks can be separated in other two types: hybrid mobile and native mobile.
These frameworks use web technologies (HTML, CSS, JS) creating a layer before to render the app called web view that is essentially a web browser.
Ionic has a great integration with AngularJS or Angular+ making sense be used by Angular developers.
These frameworks are interesting when you already have a web app and need create mobile apps quickly with equal layout and features.
These frameworks do not render in web view and do not use HTML, can be use other web technologies such as: CSS, JS, Typescript.
Today new browsers implement some or all features of the HTML5 such as: web notification, camera, file system. Perhaps your app can be a web app instead mobile app, but of course that have a mobile app is a plus.
If hybrid app is the case, IMHO "reading" the market seems that Ionic make more sense.
Otherwise, depends of your expertise, in my specific case due my experience with AngularJS and Angular+, NativeScript seems make sense but looking around seems React Native is more used. If my query is correct Google Trends proves my insight. And if you is a .NET developer Xamarin make more sense.