These settings require iPadOS 14 and iOS 14 software. To determine your software version, go to Settings, then General, then Software Update, to see if version 14 is installed.
Default Browser
Tapping a website link in an app normally opens the site in Safari - Apple’s web browser. If you prefer and have installed FireFox, Chrome, or Edge, you can force your device to use one of those browsers instead.
Go to Settings, swipe down to display the Search box if necessary, and tap inside the box. Type the name of your preferred, installed browser and tap its name when it appears. On the iPhone, a new screen opens and you'll see Default Browser about midway down. On the iPad, you'll see the same option on the right side of the screen. Tap Default Browser and choose your browser. Now, each time you tap a website link on your device, it will open in that browser.
If Default Browser does not work, check to see if there's an update available for your preferred browser.
Default Mail App
If you use the Outlook, Gmail, Hey, or Spark app for email on your device, you can choose that app as your default email app. Go to Settings, type the name of your preferred email app in the Search box, then tap the app. Tap Default Mail App and choose your app.
Caution: There was bug in these settings that forced your defaults back to Safari and the native Mail app after software updates. If that happens to you, you'll need to go through the setup process again.