Downgrade OS to Mojave

Revert your Mac to a previous macOS version - Apple Support

You did not wipe the drive as is required. How to Downgrade from macOS Catalina to Mojave or Earlier.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the partition scheme to GUID.
6. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility.
9. Insert your bootable USB flash drive and select Restart from the Apple menu.
10. Immediately, at or before the chime, hold down the Option key until the Boot Manager screen appears.
11. Select the icon of your bootable USB flash drive.
12. Click on the arrow button under the drive icon.

Did your MacBook Pro come with macOS Catalina originally? If so, then Apple does not provide a method to downgrade it to an earlier operating system. Since we don't know what MacWorld article you are referring to, would you post it please?

Finally, did you make either a clone copy or Time Machine backup of your notebook BEFORE to attempted to downgrade to Mojave?

and I expected that it would erase my HD and install Mojave.

You expected wrongly.

If you want to erase your drive before installing a macOS, you need to do that yourself.

Once booted to the installer, open Disk Utility and erase the entire device. Then, install.