Ron, when your drive structure was changed to APFS, the process created those two Volumes and you should NOT mess with them. The operating system merges them into what looks to you like one single Volume, but the Macintosh HD Volume has much higher security because it holds the system. The Data Volume holds just that, your data. But you cannot just reach in and mess with them because they are viewed as two parts of the one drive by the OS in many ways. To install Catalina, you just install it to the hardware, or the partition you created for it. It will, if needed, format the drive/partition to APFS, create the Container there, and create the two Volumes in that Container.
I'm still a bit confused by where you are right now. You say your son updated the drive in the iMac to Catalina and then tried to reinstall software but found it corrupted. What does that mean, exactly? How do you know it is corrupted? The APFS format for the drive is new and most third party disk utilities don't know how to handle it, so they report errors. How about we back up a bit to see what is really going on?