I do not think you need a new install. Did your iMac come with a Fusion Drive - an HDD and a small SSD? I need to know that before giving you a possible solution. Do I assume that you inadvertently erased Macintosh HD? If not, then how is it messed up? You have installed Catalina on an APFS formatted disk. Catalina creates a startup disk that has two volumes within the initial container. One volume, Macintosh HD, is visible on the Desktop. The second volume, Macintosh HD - Data is not visible. macOS is installed on the first volume which in turn is locked against all access including the Admin user. The second volume contains all the rest of macOS and all the users' files and data. Unix hard links are created so that the user accesses the visible volume exactly as it has always been accessed. The OS is protected from any alterations intended or unintended, making it more secure and less vulnerable to damage.
What you see in the image above is exactly what you should see in Disk Utility, but not visible on the Desktop. The two volumes share the first container which becomes identified as "disk2" even though such a disk does not physically exist. It's referred to as a "synthesized disk." There are three other volumes in the container that are also invisible: Preboot, Recovery, and VM. Recovery is the same as the Recovery HD used to repair, reformat, or reinstall macOS. The other two are used by APFS and macOS.
I need answers to my first couple of questions so I don't send you on a wild goose chase. You can downgrade back to Mojave provided you still have a full Mojave installer. Then you must erase the drive in order to reinstall Mojave. But if that's your preference, then I can show you how it's done.