Do I have a "linked" FUSION Drive and Why Does Software Update

Yes, you do have a Fusion Drive setup there. Both SSDs are linked to "disk2" which is where your system & data volumes are stored. In fact the APFS report clearly mentions a "Fusion Drive".

As for the software update issue.....since it is showing macOS 10.13 High Sierra as an option, that means you do not have a 2020 iMac since a 2020 iMac would have shipped with either macOS Catalina or Big Sur. Unfortunately I cannot identify an iMac which shipped with macOS 10.13 and supports only up to macOS Monterey. The closest option is a 2017, but that should support Ventura.

Get the system serial number being reported by macOS by clicking the Apple menu and selecting "About This Mac", then compare that serial number with the serial number printed on the base of the iMac's stand. Do the two serial numbers match? You can enter the system serial number on the check coverage page here to confirm the exact model reported by the serial number (please don't post the serial number on the forum since it is considered personal information).

Check Your Service and Support Coverage - Apple Support

Unfortunately people could potentially change the system serial number on the Logic Boards of the 2017 & 2019 models, or maybe a different model Logic Board has been installed into the enclosure. A 2020 iMac has a T2 security chip. Does macOS report a T2 security chip? You can also try booting into Recovery Mode to see if the Startup Security Utility has an option for allowing/disallowing booting from USB....I believe that is only available on a T2 equipped Mac although I am not certain. Startup Security Utility is available on non-T2 Macs.

No matter what.....if you decide to keep this computer, I would personally perform a clean install of macOS onto the Fusion Drive by first erasing the Fusion Drive followed by reinstalling macOS to ensure you have a clean copy of macOS without any nasty hidden surprises. I would suggest creating & using a bootable macOS USB installer so you have more options in case Internet Recovery Mode gives you any problems.

An iMac 2020 27" Display configured and built by Apple does not come with a Fusion Drive

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications

It comes Only with a single SSD Drive

What and How the " online reseller who "Built" the machine to my specifications " was able to make it into a Fusion Drive or even add an addition Drive into the computer is outside of my knowledge

Suggest and before were move forwards with Monterey installed and not a newer version of macOS

Would you please provide a Screen Shot from " About this Mac " without the Serial Number appearing

We are trying to establish if, In Fact , your computer is really an iMac from 2020 or some other vintage

This may, indirectly answer the question as to why the Terminal command " softwareupdate --list-full-installers " is only finding macOS 12 Monterey and nothing Newer

Ah, the joys of Third Party On-Line Custom Built / Modified Apple Computers