I am unable to delete the unknown partition on my Mac mini running macOS Monterey

Gotcha!

• When you cancel that screen does it provide any more details in the sidebar for each Partition?
• Have you used BootCamp or any other Virtual Machine?
• Do you use Xcode that shows Disk Images in the sidebar for iOS versions?

Note the Mount Point that may reveal some clue on where it may have originated and also the Device to see if it is the Partition that you are seeing. Your Partitions are likely to show in the upper part of the sidebar and not as a Disk Image.

It might give you a better understanding of your structure after the output from the following command entered in Terminal app*:

diskutil list

*If you're unfamiliar with using Terminal app (command line interface), I'm sure someone here will gladly assist.

Follow on after reading more on UTM and how to remove a UTM Virtual Machine file

Launch the UTM application on your Mac.

Locate the VM: In the UTM interface, find the virtual machine you want to remove.

Right-click (or Control-click) on the virtual machine.

Select "Delete" from the context menu.

Confirm the deletion when prompted.

Remove Image Files (if necessary):

Go to the folder where UTM stores virtual machine images. This is typically in ~/Documents/UTM or another designated directory.

Locate the corresponding image file (usually with a .utm extension) and move it to the Trash.

Empty Trash: To permanently remove the files, empty the Trash.

Those partitions in your screenshots are hidden in the main Disk Utility window because they are necessary partitions for booting an M-series Mac which is what you appear to have. Do not touch those partitions at all or you will need to perform a DFU firmware Restore to fix the mistake which destroys all data on the internal SSD.

FYI, I do not recommend anyone creating & using multiple partitions on any drive because people usually regret it later on since the user discovers one or more partitions are too small. With an M-series Mac, things are even trickier if you are trying to install another non-Apple OS on to either an internal or external drive since the internal SSD must still be modified to allow another non-Apple OS to work....things get tricky when trying to remove the other OS later on. Again, usually resulting in the need to perform a DFU firmware Restore which destroys all data on the internal SSD.

If you want to install another version of macOS onto the internal SSD, then just create a new APFS volume instead to use as the destination for the new macOS installation.

Otherwise, use UTM to run other non-Apple operating systems.

FYI, you should always provide the exact model of your device when asking for assistance as it helps to cut down on confusion. You can get the exact model of the Mac by clicking the Apple menu & selecting "About This Mac".

There Drive Structure on M-Class computer running macOS 12 Monterey

https://eclecticlight.co/2021/12/16/boot-disk-layout-in-macos-monterey/

The precise answer may depend on your specific Mac model and OS version, but...

Recent OS versions store the main OS on a read-only partition to improve performance and security (it's hard for malware to infect a read-only system). The core OS components that only really change with OS updates are stored here and the OS only unlocks the volume for an OS version update.

Other fungible data (such as user data, applications, logs, caches, etc.) live in the main 'data' partition.

Another small partition may be used for the recovery volume. This is a minimal OS version used to recover and reinstall the OS should something catastrophic occur. It's normally unused unless and until you need it, then you're thankful for it :)

Beyond those two use cases, there are other non-nefarious potential use cases. Would need more information on Mac model and OS version to be able to troubleshoot further.

UTM is a Virtual Machine ( VM ) Software.

Unlike Apple's Bootcamp, used for installing Windows on macOS, it Does Create a Bootcamp Partition

VM Software runs in Monterey. You Install the Guest Operating System into the UTM Virtual Machine Software

It does not Create a Partition on the Drive

To recapture space on the drive, you would need to look in /Users/user/Library/Containers/UTM 

Look for Large Unneeded Files associated with a Guest Opening System Installation.

To further see the drive structure.

Yes, follow the suggestion from @ishrugged 👍

Use Disk Utility found in your Utilities folder and you will have a better idea on your Partitions.