In Mojave, When I doubled clicked on my HD Icon, I would see several folders:

Applications
Library
System
Users
Data

That last folder contains all of my documents.

After installing Big Sur, the Data folder does not appear and I can't find my data. After the install I did a Migration Assistant to move all data files.

Where/how do I look for that folder?

I did a Spotlight search using the name of one of the documents and it was found. However, I have no idea where it is.

Instead of using Spotlight, in Finder, click File > Find and search there. The path to the folder should appear at the bottom of the Find window. FWIW, the Data folder doesn't show in my Big Sur installation either. I never noticed that before.

Neat way to search. Thanks.

The search results for a folder named "Data" delivered a huge list of them. I have scrolled through and found one that looks correct (It has all the folders in it that I would expect to see). However, the path does not appear at the bottom of the Find window.

The top right of the window gives choices 'Save' or delete. In the bar below, to the left, is: Search: This Mac "Desktop"

The folder is not on the desktop.

I did a bit of poking around and chose an option to "Show in Enclosing Folder". It appears in two of my backup drives but not on the main drive.

That's a personal setting: Finder > View > Show Path Bar

Okay. That works.

It shows just the backup drive and the Folder with all my data. So, how do I get the folder copied from the backup drive to the main drive?

I thought about Migration Assistant but it doesn't seem to have an option where I can choose one folder.

Why not just basic drag & drop?

That search method, of course, has been around longer than Spotlight. I guess macOS developers at Apple* decided not to prioritize making Spotlight work since it's redundant to begin with.

*Of course, as directed by corporate policy makers.

I've done that and the files are moved over except......I can't get the Data folder to where it has always been - on the Hard Drive along with the Applications, Library, System and Users folders. The Data folder sits on the Desktop and cannot be dragged/dropped onto the Hard Drive icon.