Restore Fusion Drive, bringing disk image back into hard drive

Your 2012 Mac mini apparently shipped with a 1TB Fusion drive. That was an upgraded configuration.

The images you posted appear to confirm that.

The Fusion Drive reports a size of 1.12 TB, as it should. Terabytes.

The Apple disk image Media is 2.1 GB, as it should be. Gigabytes. (You can completely ignore this disk image.)

The two volumes you saw in Catalina - "Macintosh HD" and "Macintosh HD - Data" - are normal for the new format used since Mojave: APFS. Each volume reports as being the total capacity of the drive hardware. In your case, each named volume reports its capacity as 1.1TB, so it looks like you have two 1.1 TB drives for a total of 2.2TB. However, to read it that way is a misinterpretation.

On an APFS drive, each volume shares the total capacity of the drive device, so you don't sum up the two reported totals, you simply know that the total container size is 1.1TB. Your drive size is 1.1TB.

I believe if you select the volume "Macintosh HD" in Disk Utility, then you'll see this volume is no longer an APFS volume, but a Mac OS Extended (Journaled) format. No volumes sharing drive space.

It looks to me like you're good to go here. Reinstall the macOS version of your choice and continue on, my friend.

If you reinstall Catalina, the installer will reformat the drive again to APFS, and you'll see two drive volumes again, but still only 1.1TB of total storage on the drive. That won't change.

You're welcome. 🙂