No, you are not out in the left field. You seem to understand how Lr works.
You can use the <Synchronize folder> feature - it's basically nothing different than an import.
Initially you asked: "Can I find and sync my entire pictures file at once?"
It is possible, provided that all your image folders are under one (1) parent folder. Then you could just synchronize the parent folder and all its sub-folders would be synchronized, too.
But if you don't have such a single parent folder for all image folders, you can use the Import Dialog. Select <Add> in the top center to import the images without moving them, i.e. they will be imported but remain in their present location. Then on the left side of the Import Dialog, in the <Source> panel, click on your internal drive, probably named "C". A message will appear that says "No photos found" and "Include Subfolders", click on <Include Subfolders>. This will then display all images that are on your drive in the main window. Just above the main window select "New Photos". Then only images that are not yet imported in Lr will be displayed. Then import the photos.