How many Macintosh HD volumes should there be in MacOS Big Sur?

I sent my newly purchased MacBook Pro 16" in for service (it needed a Logic Board replacement) on June 8. When I sent it in, it was running MacOS Big Sur 11.4 with only one Macintosh HD partition, at 8TB, with 4.46TB of space used. It originally came to me from the Manufacturer facility with Big Sur 11.x and just one hard drive volume (Macintosh HD). I backed it all up to my Time Machine drive just before I put in in the box for service so I'd have the very latest backup when the machine returned.

I received it back from Support servicing yesterday (June 15) with a new logic board installed-- except that it was now running macOS Catalina and had 2 hard drives partitions listed- "Macintosh HD" and "Macintosh HD - Data". I created a temporary admin account and upgraded to Big Sur, and after that I was able to restore my original account from my Time Machine backup using Migration Assistant. But I still have the two partitions ("Macintosh HD" and "Macintosh HD - Data") listed. Time Machine now won't complete a backup of my newly restored system without first renaming the drive titled "Macintosh HD - Data"; it tells me I can't have two drives with the same name ("Macintosh HD", supposedly). There is nothing of my personal files, documents, applications, etc. in the "HD - Data" drive; it's only on the "Macintosh HD" partition.

I called Apple Support and asked them if there was a way to merge the two drives into one volume called "Macintosh HD" without having to wipe and reinstall (since I'm used to having only one "had drive" visible on my machine in Finder). They insisted that the "HD/HD - Data" split is how the drives are supposed to me- even though the MBP came to me from the build factory with only one partition, like they would be pre-Catalina. They told me that the "HD - Data" volume is for my personal account folder (files, apps, documents, etc), but I told them there's nothing of that nature in the "HD - Data" partition. Also, the only applications in "HD - Data" are older Catalina-era versions of GarageBand, Pages, Numbers, etc. I told them that the MBP came to me from the factory with only one Macintosh HD partition, and they told me that was an error that would cause performance issues- all explanations that sound fishy to me.

My actual question is: How many Macintosh drives should be present under MacOS Big Sur? Should there be one drive called "Macintosh HD" or two partitions- "Macintosh HD" and "Macintosh HD - Data"? What have you all experienced? (below is the current state of my storage space after my restore from backup.)