WHen installing the OS, macOS automatically creates the data volume, nothing for you to do.

Also got a 64GB USB-C memory stick and certified it three passes over (passed), since I intend to create a bootable installer and run a fresh install when I finally do this. Also spoke with AppleCare so I'm in much better shape this far in at least, and still feel like it's taught me too much to regret any of it.

The only thing that still isn't entirely clear to me doesn't really matter since you told me there's no real need to certify the internal disk anyhow, but if I were so inclined to keep piddling away days on research, surely at this stage I could safely conduct an experiment on the (currently bootable) backup (but I don't really want to initiate a certification of a 4TB drive that arrived from OWC pre-certified, so I'm not going to). I suppose I could just delete the backup volume and try to install the OS onto the blank backup drive in Recovery Mode to simulate the experience and then maybe it would all make sense to me once and for all. 

What I'm still not sure of is how the process would look in actual practice. What I  "think" after all of this....is that I'd just load Disk Utility, select the blank drive (which would contain no containers because it would contain no file system format with no volumes at all on it post-certification), hit "Erase," give it a name (like Mac HD) and select it to be APFS, which would allow me to reload the macOS onto it. But it's still not clear to me if the second container (the Data container) would be created automatically upon installation of the operating system.

In any event, we're talking about a new MBP from Jan. 2021 with no indication of or concern for drive issues, so of course there's no need for me to really do this. But understanding is enough for me sometimes. I'd rather understand more and accomplish less than accomplish more and understand less. To a fault at times (as evidenced by Day 4 of something that most people would have just installed from the Finder and moved on with their lives). 

And, upgrade SoftRAID to 6.3. SoftRAID is a disk driver, so keep it up to date. Good luck!

I thought I mentioned you cannot install Catalina onto a SoftRAID volume. And, SoftRAID volumes are not bootable any longer.

With newer OS's, no RAID can be bootable, including Apple's.

It is sometimes possible to make a SoftRAID 5 volume bootable by cloning Catalina, but difficult and I don't recommend you try.

Stick to normal startup volumes and back up your data.

It has come to my attention that SoftRaid 6.1 adds APFS. As I am currently behind on version 6.03, of course I need to update my SoftRaid, but I guess now I'll be updating everything and hopefully never getting this far out into outer space ever again. I have learned a lot in the process, at least 

@guitarflex 

You are getting problems, so keep it simple.

initialize your extenal with Apple Disk Utility, startup with command r, and reinstall macOS, and point it to your drive.

DOn't bang your head against the wall when it hurts. Do the Apple approved way. If that does not work, nothing is likely to.

We have major progress but now a very specific SoftRaid question. Just spent 45 minutes on the phone with AppleCare which sorted out the installer issues and I very conclusively do now have an installer for Catalina (to get a bootable backup).

I cloned the computer to the ThunderBay slot 1, Aura P12 NVMe SSD. CCC help says to install the macOS to the backup drive. When I open the installer now, it tells shows the external volume but is greyed out with a note "You may not install to this volume because it is part of a SoftRAID." The volume was created as a non-RAID. 

I can certainly use Disk Utility instead and start over, but wanted to ask here before erasing the clone and starting over via DiskUtility. Thanks again for the advice. I realize this is taking longer than expected and wish to express my gratitude and appreciation for bearing with me through the process.

You are getting problems, so keep it simple.

initialize your extenal with Apple Disk Utility, startup with command r, and reinstall macOS, and point it to your drive.

DOn't bang your head against the wall when it hurts. Do the Apple approved way. If that does not work, nothing is likely to.

@guitarflex 

Catalina on intel should be possible to boot it.

Erase it again with Disk utility.

See if you can command r restart, and install it from there?

Just to make sure I'm understanding (and to clarify any confusion I may have caused before!), are you talking about the bootable backup via CCC, or are you specifically referring to a bootable Mac OS installer drive/disk/stick? Right now I'm just trying to get my CCC Backup to be bootable. This may necessitate reaching out to the folks at CCC although the help article says I should be able to just download the installer and install it onto the backup disk, which is where I got stuck last night. The Catalina installer didn't download from the App Store but I'm working on re-trying today. I found a long webinar on YouTube linking to a GitHub page where you're supposed to be able to download installers but I'm not sure I want to take a chance with anything that doesn't come straight from Apple so it's back to trying again today. Will keep posted. At this point, obviously it's a CCC support issue but I do thank you for the ongoing help/suggestions/guidance. To be continued.

(from CCC:)

"If your data-only backup resides on a non-encrypted APFS volume, you can install macOS onto the backup disk to make it bootable.

Intel-based Macs

1. Download and open the macOS Installer:
2. When prompted to select a disk, click the Show All Disks... button and select your backup disk
3. Proceed to install macOS onto your backup disk"

Catalina on intel should be possible to boot it.

Erase it again with Disk utility.

See if you can command r restart, and install it from there?

@guitarflex 

run this in terminal:

csrutil status

If it says enabled, no problem.

Ran that in terminal. It says enabled. Hopefully this reduces/eliminates the chance of what you said earlier that: "future macOS systems may quarantine the SoftRAID driver." 

Will have to keep you posted. I seem to have confirmed that my Carbon Copy Clone (to the Aura P12 Pro over TB3 -- it was blazing fast to clone) is not currently bootable, so it wasn't as simple as the slow USB-A drive not being recognized over a hub. Once I tried the clone to the ThunderBay and allowed external devices in Startup Security Utility, the computer still doesn't recognize an external device to boot from. I was reading about this on the Carbon Copy Cloner page/site and assume that it's truly not creating a bootable backup now. They say that in order to make a Standard Backup bootable, to install macOS onto the backup disk to make it bootable. However, after downloading a Catalina installer (to make it a Catalina bootable disk, as I'm still on Catalina), I got an Apple notice: "Installation failed. An error occurred while installing the selected updates."

I have not tried since but it's 1:30 am now and perhaps tomorrow will see some steps forward. Right now I'm a little fried from the past few days! Thanks for your help and support.