How to create Catalina.app for creating bootable disk
If one is already running macOS Catalina, just open Terminal and use the following command to download the latest release of macOS:

softwareupdate --fetch-full-installer


Or to download a specific version, even older, use e.g.:

softwareupdate --fetch-full-installer --full-installer-version 10.14.6


To create a USB installer, then in Terminal use the createinstallmedia command, e.g.:

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume



See How to create a bootable installer for macOS for details. 



For softwareupdate internal help output: 

% softwareupdate --help
usage: softwareupdate &lt;cmd&gt; [&lt;args&gt; ...]

** Manage Updates:
    -l | --list     List all appropriate update labels (options:  --no-scan, --product-types)
    -d | --download     Download Only
    -e | --cancel-download      Cancel a download
    -i | --install      Install
        &lt;label&gt; ... specific updates
        -a | --all      All appropriate updates
        -R | --restart      Automatically restart (or shut down) if required to complete installation.
        -r | --recommended  Only recommended updates
    --fetch-full-installer      Install the latest macOS Installer
        --full-installer-version    The version of macOS to install. Ex: --full-installer-version 10.15
    --background        Trigger a background scan and update operation
    --ignore &lt;label&gt; ...    Ignore specific updates
    --reset-ignored     Clear all ignored updates

** Other Tools:
    --dump-state        Log the internal state of the SU daemon to /var/log/install.log
    --evaluate-products Evaluate a list of product keys specified by the --products option 
    --history       Show the install history.  By default, only displays updates installed by softwareupdate.  
    --all           Include all processes in history (including App installs) 

** Options:
    --no-scan       Do not scan when listing or installing updates (use available updates previously scanned)
    --product-types &lt;type&gt;      Limit a scan to a particular product type only - ignoring all others
        Ex:  --product-types macOS  || --product-types macOS,Safari 
    --products      A comma-separated (no spaces) list of product keys to operate on. 
    --force         Force an operation to complete.  Use with --background to trigger a background scan regardless of "Automatically check" pref 

    --verbose       Enable verbose output
    --help          Print this help
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