Fractal is a web-based application and is designed to be operated within a browser environment. As such, integration with mobile apps is currently only achievable using deep linking mechanisms.
The integration flow is essentially the same, whether the user's starting point is your web application or your native app (see User authorization for more details):
You redirect the user to Fractal
The user goes through the Fractal journey
Fractal redirects the user back to your application
The redirection in point 3 is towards the redirect_uri you configure in your integration (see Getting started for more details). If you want this redirection to happen towards your native app, instead of your web application, you need to make use of the operating system's deep linking mechanisms: Android App Links in Android, and Universal Links in iOS. They allow you to register particular URLs as URLs to be opened inside your native application instead of the browser.
Once these are set up, the user will be redirected to your app at the end of their journey in Fractal.