You should be able to visit the Update panel on the Mac App Store application (provided you are signed into the Mac App Store), and click to download any available updates for these particular applications. For Catalina, you should have Pages v8.2.1, Numbers v6.2.1, and Keynote v9.2.1. You can verify this version info from the Finder Get Info panel on each application.
The first time that you use a new version of Pages, it will pop a what's new panel on you.
If you have a new iMac with Pages, Numbers, and Keynote pre-installed, you must acknowledge them in the Mac App Store by clicking on your avatar/name in the lower left corner of the opened Mac App Store application. This will take you to a screen where you can confirm/download if necessary these applications. This is the equivalent of the Purchase tab that existed prior to Mojave.