When you tap a link in your emails using Outlook Mobile, you can choose which browser you'd like to use between Microsoft Edge and your system default browser.
If you'd like to save your preferences, turn on the "Remember my choice" toggle in the card shown. You can also change your preferences by going to Settings > Open Links With.
If you have a work or school account in the app, such as a Microsoft 365 or Exchange account, your IT admin can manage this setting. If your admin has hidden it, you will not see the Open Links With option in Settings.