Merge partitions / volumes

To follow on @Barney-15E suggestion >>

This method will WIPE ALL Data and there is No Recovery - Period.

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Only an Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Once that is done read on for preparing the Destination computer  >> Only works on Intel Based Apple Computers

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above. 

5A - How to fix a split Fusion Drive

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

I agree with Grant, you have a Fusion drive within s a small (100GB in your case SSD) integrated with a large (1TB HDD in your case). The drives are separate but operate as one unit with the macOS putting frequently used files on the SSD for speed.

You can split the Fusion drive into its tw drives but a 100GB SSD is too small to be used as your startup drive and just using the 1TG HDD as your startup drive would result in a slow Mac.

You can get a fast USB-C SSD and use that as your Startup drive which a lot of iMac users have done when one component of the Fusion has failed.

If you have the iMac taken apart and an large SSD installed internally instead of one of the existing drives. Taking an iMac apart to do so is not that easy. You can go to iFixit and see yourself.

Might be a stupid question but since I'm not advanced user I will ask. What are the risks of carrying out this fix as per link above?

What might be the risks of entering all of your financial information on that computer not knowing what the previous owner installed on it?

Will I be able to revert to current setup (or original Catalina) if something will go wrong? Last thing I want is to have problems after spending lots of money.

I thought this was a new (to you) computer. Have you already put data on it that you want to preserve?

I would suggest saving anything you have created to an external drive so you can copy it back when you complete reinstallation.

You can install any OS that will run on that Mac. Just download the installer from here, then create a bootable USB installer following the specific instructions, here. Use that bootable USB to erase, repair, and reinstall the OS.

You can install Mojave or later on that Mac. I wouldn't bother with Mojave. You should be able to run Monterey without issue.

The previous owner may have had some software that wouldn't run under Big Sur or Monterey. That's the only reason I can think of to keep Catalina.

A Fusion Drive is a merged SSD/HDD. The OS and heavily used files (apps) are stored on the SSD while other data is stored on the slower HDD. The OS manages that all in the background.

If you have any questions on those two articles, please ask before pressing forward. We may not get back to you immediately, but usually within a day.

You should fix the Fusion drive (already linked, earlier). That will erase both SSD and HDD. The original owner likely split it because that setup worked horribly on Monterey. I believe that problem was fixed.

Regardless, any appropriate solution involves you erasing and reformatting since the original owner did not do that.

What are on the different disks?

1. It appears that the first disk is your startup disk and it has two volumes which is normal and required for a startup disk. See: https://eclecticlight.co/2021/12/16/boot-disk-layout-in-macos-monterey/ Nothing you can combine here
2. Is there a volume for Container disk 4?
3. What values are on untitled>Container disk 3? The disk utility lists Snapshots and those are typically associated with a backup like for Time machine.
4. What does Finder show about the second drive (disk 4 and 3)?

As long as the Fusion drive is working as designed then there is nothing that should be done. Wirth Fusion drive you have the disk structure/layout as designed and it can't be changed.

If in the future you have problem with one of the drives that make up a Fusion drive, the least expensive "fix" that has acceptable speed is to get a fast USB-C SSD and use that external drive as your startup drive.

EDITED

I stand to be corrected my a more learned contributor and accept that as possible.

The Drive Structure is fine.

The issue is, the Previous Owner had Catalina macOS 10.15 installed and then Added a Container after Catalina and installed Monterey into that Container.

This effectively makes it a possibly Duel Boot setup.

Option Boot the computer and choose Catalina or Option Boot Monterey.

There is where it gets picky.

If Catalina macOS 10.15 is in the First Container - Catalina can not be removed. Added, even if Catalina were possible to remove and being the First Container - the used space would Not Become Available to be reused by Monterey.

If the Monterey was first, yes the Catalina could be remove and the space would automatically be usage in Monterey.

Open to corrections and insights

Installing macOS on a separate APFS volume this is what appear the previous owner may have done

EDITED

Judging from the images of the OP - this appears to be a Fusion Drive if this is of any really importance.

SM000032L Media and APPLE HDD ST1000DM003 Media

Q - " Just in case it is helpful, I have no data stored on this Mac so I don't need to back up anything. "

A - " Pervious own computer " should have done this before putting it up for Sale

What to do before you sell, give away, or trade in your Mac

Personally, would return it to previous Owner and them perform this action, as well as putting the Fusion Drive back to gather as One Partition / Volume

If that is not doable - guess it falls to the new Owner

There is a way to Remove the Monterey and be left with Only Catalina.

Then, just need to upGrade the computer to / and suggested to Big Sur macOS 11 first and then move up to Monterey macOS 12.

The extra step is from Observations on these Forums.

Jumping Directly from Catalina, although possible - sometimes does not work right.

Some have been successful and other have had a packet of issues

" I agree with Grant, " hum :-(