This article applies to Arc on macOS and Windows.
Your default browser opens automatically anytime you click a link in another app.
macOS
To set Arc as your Default Browser, go to Arc > Set as Default Browser in the Mac Menu Bar:
Windows
To set Arc as your Default Browser, go to Arc menu > Set Arc as Default Browser.
This will open Windows Settings > Apps > Default apps > Arc. Click the Set default button to set Arc as your default browser.