While Apple no longer highlights Catalina on its main download page, the official DMG-like installer is still available. Here’s how to download the genuine file safely. Strictly speaking, Apple does not provide a raw .dmg file of the full macOS Catalina installer. Instead, they provide an Install macOS Catalina.app inside a .dmg (or as an app downloaded directly from System Settings).
If you’re looking to perform a clean install of macOS Catalina (10.15) or create a bootable USB installer, you’ll need the actual Install macOS Catalina.app — not just the small stub installer from the App Store. download macos catalina dmg