Apple sometimes hides legacy installers behind support article links. As of 2025, this method works inconsistently but is worth trying.
Apple Combo Update (if you only need to update an existing Catalina) download macos catalina 10.15.7 full installer
To download the full installer for macOS Catalina 10.15.7, follow these steps: download macos catalina 10.15.7 full installer
Notes on official sources:
Before installing macOS Catalina, make sure your Mac is compatible with the operating system. You can check Apple's website for a list of compatible Macs. download macos catalina 10.15.7 full installer