how to download macOS Catalina without installing

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Catalina would have to be done on a computer that Qualifies to run Catalina.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

You are welcome and thank you too.

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise

And in future - refer to post just above this one.

Once downloaded and it attempts to Launch - Quite the Installer

Once the installer downloads & opens, just quit the installer without allowing it to continue. The installer will remain in the Applications folder on your system drive.