Since the earliest days of the iPhone, Apple’s Safari and Mail have been the default Web and email apps for iOS and iPadOS. There was no way to choose alternatives that would be used whenever an app wanted to open a Web page or create an email message. That changed with iOS 14 and iPadOS 14, which introduced a way to switch to a different Web browser (such as Brave, DuckDuckGo Privacy Browser, Firefox, Google Chrome, Microsoft Edge, or Opera Touch) or a different email app (such as Boomerang, Chuck, Hey, Gmail, Outlook, Polymail, or Spark).
In the Settings app, tap the name of the browser or email app you want to set as the default. Then tap Default Browser App or Default Mail App and select the desired app.