How to install Pages on macOS Catalina version 10.15.7?

If you never installed Pages since Oct 2013 via the Mac App Store, then you will need Monterey or Ventura to install the current version, and there is no means to get Pages for Catalina.

If you did install it previously, then launch the Mac App Store and sign in with the same Apple ID as you used previously to install it. Press cmd+0 (zero) to open the Purchases panel, and Pages should be there with an adjacent blue outline cloud icon. Click that and it will offer to install the last compatible version (11.1) of Pages for Catalina.

Pages now requires macOS 12.0 or later. If you have downloaded Pages previously, you can still download an older version from the App Store, otherwise you can't.