If you need an app to view excel type files, you can use the free & open source LibreOffice app. If you downloaded or "purchased" Apple's own Pages & Numbers, etc. apps from the App Store for macOS Catalina before now, then you can use those Apple apps with the Microsoft files like excel files. If you did not previously "own" the Apple Numbers app while Catalina was the current OS, then you won't be able to download a version of Numbers compatible with macOS 10.15 Catalina now. You would likely need to check "Previous Purchases" to get a download link, but only if you had installed it back then.
Unfortunately Microsoft's file formats are poorly designed & documented, there will be some compatibility issues using non-Microsoft apps to view files created with Microsoft Office apps.