High Sierra and Mojave both had warning panels when opening 32bit apps that they would not be operable in newerOS's, this is the big difference in Catalina, how did you not notice.
If you want to ditch Catalina and go back to a previous OS read the following article
Revert your Mac to a previous macOS version - Apple Support
If you have access to another mac that is able to run High Sierra or Mojave you can download whichever of these two OS's from these links, (this donor mac must not be running Catalina)
How to upgrade to macOS High Sierra - Apple Support
How to upgrade to macOS Mojave - Apple Support
Follow the instructions in Section 4 to download the OS you want.
Once downloaded you can then create a bootable USB stick using the instructions here,
How to create a bootable installer for macOS – Apple Support
Having created the bootable USB installer plug it in to your Catalina Mac, make sure your external drive is connected Restart while pressing and holding the option/ alt key. Your mac will now boot to the Startup Manager where you select the USB and press Return.
When you boot to the USB you should see a Utilities window, select Disk Utility,
click on View in the menubar and select Show All Devices.
In the left hand column you should see your external drive, under the External section.
Highlight the Disk not any indented Volumes. Click Erase,
give the disk a name,
Format: APFS.
Scheme: GUID Partition Map
click Erase.
Having now formatted the external disk you should be able to install the OS of your choosing.
