"So, if I understand you correctly, since my top LR folder is named _Lightroom_ (to make it visually different to me from other folders), all I need to do is periodically right click on that folder - and LR will take over and make sure everything under that folder will be included in the folder list for LR, even if there are folders there that don't contain any images that have been imported into Lightroom? ....and when I do right-click on it, what do I do next? None of the choices seem to indicate it will do what you suggested?? Am I missing something?"
The term "folder" has some ambiguity. On the computer hard drive, folders hold groups of files and these can be visualized and manipulated using the system file browser (Windows Explorer in Windows, or "Finder" in Mac I think). If you have a computer disk folder named "_Lightroom_" with sub-folders under it, that can be useful. Right click on a hard drive folder viewed in a system file browser may bring up a system contextual menu with system choices (copy, delete, move, etc.) but no Lightroom commands. The folders on your computer drive can contain images, videos, documents, spreadsheet files, and so forth. You can use various programs (including Lightroom, Photoshop, OnOne, etc.) to edit the image files and save the edited versions back to the hard drive in the same folder if you like.
In Lightroom, the catalog "folders" seen in the Library module over on the left hand side are a representation of what the Lightroom catalog "knows" about your hard drive folders and files. If you right click on one of these catalog "folders" showing within Lightroom, you will get a Lightroom contextual menu. One of the choices will be to "Synchronize Folder..." Clicking on that will bring up a small dialog box with some choices. It will count the number of images not yet in the catalog and let you check a box to import them into the catalog. It will let you check a box to scan for any metadata changes and import the new information. If you synchronize from high up in the Lightroom folder hierarchy, Lightroom will execute a new read of that folder and all associated sub-folders and then update the Lightroom catalog. If you synchronize from a low level of the hierarchy, Lightroom will execute a new read of just that part of your system (which may be faster and might be all you need if you just used PM to put new images into a single new hard drive sub-folder).
When Lightroom synchronization detects an image file (and at least some sorts of video files), the "synchronize folder" command will add those files to the Lightroom catalog database whether the images have been edited in some other program or not. Once imported into the Lightroom catalog, you can use Lightroom to edit the files. Of course, even after addition to the Lightroom catalog, you could still find those files in your computer hard drive folders using other programs to further edit those images if you wish. Just remember that if you do that, some changes (like keywords added) might be invisible to Lightroom until you synchronize the catalog folder(s) again.
Victoria Brampton, known as the "Lightroom Queen" has a web site where she sells eBooks that do an excellent job of explaining all the mysteries of Lightroom. Worth considering as a way to accelerate getting the most out of that program.
Edited to add a disclaimer that my description applies to the "Lightroom Classic" desktop version. The Lightroom CC version using the cloud for storage might have different features. Also, the "Import" function in Lightroom is sometimes more appropriate than "file synchronize," depending on what you need to do.