You can find the disk image on the Apple Support macOS download page .
(Rated not for features, but for utility and necessity in the modern era)
Instead, Apple provides an InstallMacOSX.dmg (which contains the .app installer) or a direct .app file. Once you run that package, the full OS installer is extracted.
Apple still hosts the official DMG on their servers. This downloads a file named InstallMacOSX.dmg .
This is where the review turns frustrating. Apple has a habit of hiding older OS installers.