In iOS 14 and later, users can select an app to be their default web browser or email app. Apple has posted a developer document on how to prepare an app to be the default browser or email client.
To make your app a choice, developers must confirm that it meets the requirements in the doc, then request a managed entitlement.
The system invokes the default web browser in iOS whenever the user opens an HTTP or HTTPS link. Because this app becomes the user’s primary gateway to the internet, Apple requires that web browsing apps meet specific functional criteria to protect user privacy and ensure proper access to internet resources.