Finding a direct URL for a macOS installer is notoriously difficult because Apple hides these links behind the App Store frontend. The "proper" method to obtain the DMG involves using the softwareupdate tool built into the Terminal.
To generate a direct download for Catalina, users can utilize the following command in Terminal:
softwareupdate --fetch-full-installer --full-installer-version 10.15.7 Macos Catalina Dmg Direct Download
This command forces the Mac to contact Apple's servers directly and download the complete "Install macOS Catalina" application into the Applications folder. While this downloads the installer, converting that into a DMG requires an additional step using the Disk Utility or the hdiutil command to create an image of the installer volume. Finding a direct URL for a macOS installer
Apple’s catalog URL can be used, but it requires parsing XML.
Simpler: use MDS (Mac Deploy Stick) or installinstallmacos.py: Then select Catalina 10
curl -O https://raw.githubusercontent.com/munki/macadmin-scripts/main/installinstallmacos.py
python3 installinstallmacos.py
Then select Catalina 10.15.7 from the list.
Catalina replaces iTunes with three apps. To fetch back the “Device” sidebar: