2 Mac Data volumes

Good question

The information below should work and here is the link for reference Mount a volume that’s been ejected using Disk Utility on Mac - Apple Support (CA)

If you eject a volume, you can use Disk Utility to mount the volume so that it’s available in the Finder.

1. In the Disk Utility app  on your Mac, select the volume you want to mount.
2. Click the Mount button .

Great work and I see the issue

1 - Suggestion is to Mount the Volume marked in below image and copy any Important Personal Data OFF that volume to say an External Drive.

2 - Once done, that same volume in below image and be Removed

3 - Just Right Click the volume with the " Volumes/Macintosh HD - Data " in the image and as Sub Menu should appear >> Delete

4 - The Data in step 1 above can be moved be moved back to your Regular Data Volume if required

We are of similar age and am confident with a little hand holding ( if I may ) say we can get this fixed.

In the Disk Utilities >> View >> View All Devices as per image below.

If you can use the Keyboard triple keyboard combo Shift Command 5, it will offer to make a Screen Shot of what is presented in Disk Utilities

Once we have that info, we can move forwards

When the User ( you ) choose the Reinstall Big Sur, the user may have inadvertently created a New User Account.

This would have left the then existing User Account still on and in the " Volumes/Macintosh HD - Data " as denoted by the Mount Point in bold above and within the " and "

This does occur from time to time but is fixable by the actions suggested.

If you are sure there is nothing of Value on that Volume - just remove it.

That will work 👍