Format SSD drive and reinstall Mac OS Catalina

Because you mentioned that the first link has gone bad, for reference to others: I have copied the entire support article: Add, delete, or erase APFS volumes in Disk Utility on Mac

Apple File System (APFS) allocates disk space on demand. When a single APFS container (partition) has multiple volumes, the container’s free space is shared and can be allocated to any of the individual volumes as needed. Each volume uses only part of the overall container, so the available space is the total size of the container, minus the size of all volumes in the container.

Add an APFS volume

Each volume you add to an APFS container can have a different file system format, if needed.

1. In the Disk Utility app  on your Mac, select an existing APFS volume in the sidebar, then click the Add Volume button  in the toolbar.
2. Enter a name for the new APFS volume.
3. Click the Format pop-up menu, then choose an APFS format.
4. If you want to encrypt the volume, choose APFS (Encrypted) or APFS (Case-sensitive, Encrypted).
5. If you want to manually manage APFS volume allocation, click Size Options, enter values in the fields, then click OK.

• • Reserve Size: The optional reserve size ensures that the amount of storage remains available for this volume.
  • Quota Size: The optional quota size limits how much storage the volume can allocate.

1. Click Add, then click Done.

Delete an APFS volume

When you delete a volume, all the data on the volume is permanently erased, and the volume is removed from the container.

1. In the Disk Utility app  on your Mac, select the APFS volume you want to delete in the sidebar.
2. Click the Delete Volume button  in the toolbar.
3. Click Delete, then click Done.

Erase an APFS volume

When you erase a volume, all the data on the volume is permanently erased, and the empty volume remains in the container.

1. In the Disk Utility app  on your Mac, select the APFS volume you want to erase in the sidebar.
2. Click the Erase button  in the toolbar.
3. (Optional) Enter a new name for the volume.
4. (Optional) Click the Format pop-up menu, then choose an APFS format.
5. If you want to encrypt the volume, choose APFS (Encrypted) or APFS (Case-sensitive, Encrypted).
6. Click Erase, then click Done.

You can’t delete or erase your startup volume. See Erase and reformat a storage device.

See also

About Disk Utility on Mac

File system formats available in Disk Utility on Mac

Those APFS volumes were, most likely, created by the previous owner.

Here's how you can delete them:

Add, delete, or erase APFS volumes in Disk Utility on Mac

Yes, you can re-install the OS again.

How to reinstall macOS from macOS Recovery

If you want to nuke the drive (and all its files!) and do a clean install:

Download Catalina installer and copy it to a flash drive.

https://support.apple.com/en-us/HT201372

Then shutdown and Option-boot (or set the flash drive as a startup disk) into the flash drive (I guess your iMac does not have a T2 chip so enabling external booting should not require extra steps), launch Disk Utility, make the internal device visible at the left so you can select it (not the container or volumes under it), format it as APFS with GUID partition scheme, quit Disk Utility and enter the Catalina installer and choose to install on the freshly formatted internal disk.

Great! Good job! I'm glad it worked out for you!

Enjoy ;)