How to Test Apps and Browser Extensions

I wanted some advice from a business cybersecurity standpoint;

How can I scan and test applications for viruses and other malicious activity from the likes of the App Store, Google Play Store, and Microsoft App Store? This is for both mobile and desktop apps.

I also wanted to extend this question, and ask if there is a way to do the same sort of scanning/testing for browser extensions to make sure they're not also malicious.

Thanks :)

apps browser browser extensions cybersecurity extensions test