So the previous 3rd-party WIDGETS I had were lost in the upgrade, and, presumably, those 3rd-party WIDGETS can no longer be downloaded to the existing application package.
Yes and yes.
Is it fair to conclude that ONLY the WIDGETS which come bundled with the Apple OS are all that are available to the iMAC user?
Any application developer can include coding for a widget in their application. I have no idea how many third party application developers have added widgets to their application but I currently have three apps (Forecast Bar, Mind Node, & Network Kit) providing widgets in addition to those included in the Big Sur installation package. Network Kit alone has five widgets, one in two size options, and Mind Node has three. Only Network Kit makes any mention of Widgets in their App store description so to my knowledge there is no reliable way to tell if an app offers widgets or not. As to their usefulness I find the new widgets to be far more useful than the old type widgets ever were to me. In fact, they have changed the way I use my iPhone and they are changing the way I use my Mac.