Mojave rollback with a super inexperianced mac person

There is an exception in your case... Pre-Installed MacOS:X 10.14.5 (18F132) Maximum MacOS:Current

https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i5-2.4-quad-core-13-mid-2019-touch-bar-specs.html

Trouble is getting Catalina to Agree to it.

Don't use FireFox for this link…

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Use the 'createinstallmedia' command in Terminal

1. Connect the USB flash drive or other volume that you're using for the bootable installer. Make sure that it has at least 12GB of available storage and is formatted as Mac OS Extended.
2. Open Terminal, which is in the Utilities folder of your Applications folder.
3. Type or paste one of the following commands in Terminal. These assume that the installer is still in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If it has a different name, replace MyVolume in these commands with the name of your volume.

1. Mojave:*

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

Hopefully then holding option key at power up give you the option to boot from the USB Installer &can Install to another external drive.

if your computer came new with Catalina you can not revert to Mojave.