On Catalina, you can find it in the App Store (https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12 https://apps.apple.com/us/app/macos-big-sur/id1526

On Catalina, you can find it in the App Store (https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12 https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=...) but when you click 'Get' it moves over to Software Update. I got the download started after a few tries but it is crawwwllling.