After 5 hours wasted due to incomplete nopCommerce documentation, I am sharing what will likely be helpful.
The installation documentation seems to be missing 2 entries that need write permission:

\App_Data\DataProtectionKeys\
\App_Data\appsettings.json


Additionally, if you have customized directory structure permissions, "CREATOR OWNER" needs write permission for any of the listed directories (or add it at the root of the site if inheritance is enabled).

Environment:
Windows Server 2019 Std, joined to AD domain, websites directories are NOT on the C:\ drive.

Information that will help this get found, Application Log error:
------------------------
Source: .NET Runtime
Event ID: 1026
Application: w3wp.exe
CoreCLR Version: 5.0.421.11614
.NET Version: 5.0.4
Description: The process was terminated due to an unhandled exception.
Exception Info: System.UnauthorizedAccessException: Access to the path 'X:\InetPub\NopSite\V440\App_Data\appsettings.json' is denied.  [snip]
------------------------
Kudos to @razor777, gave me the idea to add Everyone - Write permission to the root level (on a non-live site!), that allowed the site to finally load, and (with "Owner" column showing in explorer) it was dead-easy simple to see what files/folders that nopCommerce app needed.  Reference:
https://www.nopcommerce.com/en/boards/topic/55568/systemexception-plugin-must-be-assigned-to-customer-role-access-to-the-path-is-denied

Reference:
https://docs.nopcommerce.com/en/installation-and-upgrading/installing-nopcommerce/installing-local.html

Therapy:
Yes, 5 hours wasted due to this being a migrate from V3.90 to 4.40, the site would error with unhelpful message:
HTTP Error 500.30 - ASP.NET Core app failed to start
It seemed like there was a problem migrating the old InstalledPlugins.txt and Settings.txt files, but it was impossible to tell if V4.40 would do this automatically after having skipped several upgrade versions. (I STILL don't know that answer!)

Eventually, after triple and 5x rechecking permissions on the app, the pool, the files, (and under my desk) I decided to try a fresh install of nopCommerce.  I still had the same 500.30 error at launch.  That is when I came across the suggestion to add Everyone - Write permission.  Bada-bing the test site loaded as it should have.  From there I had to figure out what files were written with the nopCommerce app pool user (I always have the "Owner" column showing in File Explorer by default).

I admit my customers haven't upgraded in a while, so I was a bit rusty with setup.  But I don't recall ever having this amount of difficulty finding accurate nopCommerce documentation.

Suggested action items:
- Update the documentation to include the 2 additional items.
- Add a note to the Release Notes indicating if InstalledPlugins.txt and Settings.txt files will be automatically migrated to .json files.