On a Mac, you can use this command to download the latest available Catalina installer: softwareupdate --fetch-full-installer --full-installer-version 10.15.7 System Requirements for macOS Catalina
If you are already running an earlier version of Catalina, you can download the macOS Catalina 10.15.7 Combo Update (4.82 GB) directly from Apple Support. Mac Os Catalina 10.15.7 Download Dmg