folders on hard drive not showing up in LR classic

Ahh, NEW iMac.

Outside of Lightroom, on the iMac, bring up System Preferences (MAC) select Security and Privacy, select the Privacy tab, scroll down to Files and Folders, make sure a Lightroo Classic and while you are at it Photoshop have rights to everything.

Also, that section on Full Disk access, something in their I am forgetting about.

also, for reference, see:

https://laurashoe.com/2019/10/15/whether-and-when-lightroom-users-should-install-macos-catalina/

https://www.lightroomqueen.com/lightroom-macos-catalina-compatibility/

and about that Full Disk access, perhaps a special case, see : https://community.adobe.com/t5/lightroom-classic/problem-lightroom-classic-would-like-to-access-files-in-your-documents-folder-os-x-catalina/m-p/10772414

In the import screen, under Sources, are you looking in Devices (throwned upon if even visable) or Files?

Lightroom is not a file browser that will automatically display every folder that exists on your hard drive. The only folders that will display in the library are the ones that you have deliberately imported using the import feature in Lightroom. If the images have not been specifically imported using the Lightroom import feature they will NOT display in Lightroom at all. Lightroom utilizes a catalog, which is a database. The only way to get folders added to that database is through proper use of the Lightroom import function.