To install macOS Catalina 10.15.7:
Apple no longer shows Catalina in the main "Software Update" pane unless you are already running Mojave or earlier. For newer Macs, you need the App Store link. macos catalina 10.15.7 download
If you need for compatibility with older 32-bit apps or legacy hardware, here is the official way to get it from Apple. To install macOS Catalina 10
To ensure a smooth installation, verify that your Mac meets the minimum system requirements for macOS Catalina: macos catalina 10.15.7 download