When entering the command it said “command not found” I am trying to download macOS Mojave and am running macOS Catalina

The Mac App Store knows that your Mac can run Catalina, and will block your download of Mojave — if you are using the Mac App Store link to the full Mojave installer from How to get old versions of macOS. There are no full installers on Apple's download site.

You will need a Mac that is currently running Mojave, or running an older operating system where the hardware can upgrade to Mojave.

Mojave  macOS 10.14 via Terminal

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled

Where the " Untitled " in above command is the Name Given to the destination drive where the USB Bootable Installer is to be made.

I have created a Catalina USB startup drive per the article. I'm not sure about using it. I booted from it and I was asked which disk I wanted to install it on. I don't want my mac to startup from the Catalina OS permanently, I just want to use it for certain apps. If I download it to my hard drive as I am being asked to and it installs there will that become my default OS?. I prefer to use Mojave, so I don't understand what to do.

Generally command not found means a missing space before or after the command.