How do I download Keynote for Mac Catalina?

Launch the Mac App Store, and sign in with the same Apple ID as you have used in the past to install Keynote. Press cmd+0 (zero) to open the Purchased panel and you should discover Keynote with blue cloud and download arrow next to it. Click that, and you should receive a single panel offering to install the last compatible version of Keynote (v11.1) for macOS Catalina.

If you have never installed Keynote on any other Mac with your Apple ID, then you cannot get it for Catalina.