Source: https://support.apple.com/ru-ru/HT211683
It is also possible to fetch some of these via command line:
/usr/sbin/softwareupdate --fetch-full-installer --full-installer-version 10.13.6
/usr/sbin/softwareupdate --fetch-full-installer --full-installer-version 10.14.6
/usr/sbin/softwareupdate --fetch-full-installer --full-installer-version 10.15.7
sudo "/Applications/Install macOS Big Sur.app/Contents/Resources/createinstallmedia" --volume /Volumes/MyVolumeName
Source: https://support.apple.com/ru-ru/HT201372
More links: https://support.apple.com/en-us/HT211683