If the App Store method fails or you want a specific build, you can use the softwareupdate command in Terminal. How to download and install macOS - Apple Support
Getting macOS Catalina 10.15 (Build 19A583) on Any Mac macOS Catalina 10.15 (Build 19A583) marked a significant shift for Apple’s desktop OS as the first version to drop support for 32-bit apps and introduce Sidecar. Whether you are looking for the original 10.15.0 release or need to "patch" it for an older, officially unsupported Mac, there are a few specific ways to get the installer today. 1. Download the Full Installer from the Mac App Store
: Once the page opens, click the "Get" button. This will trigger the Software Update pane in System Preferences to begin the download.