macOS: Download previous versions of macOS

macOS 10.15 Catalina
https://support.apple.com/en-us/HT201475

macOS 10.14 Mojave
https://support.apple.com/en-us/HT210190

macOS 10.13 High Sierra
https://support.apple.com/en-us/HT208969

macOS 10.12 Sierra
https://support.apple.com/en-us/HT208202

OS X 10.11 El Capitan
https://support.apple.com/en-us/HT206886

OS X 10.10 Yosemite
https://support.apple.com/en-us/HT210717