The Previews folder contains all the previews that get shown in the Library Module within LR. You're right, it's a large, mostly empty directory tree. This is normal and is by design.
Double-clicking on the .LRCAT (not .IRCAT) file opens Lightroom with that file as the catalogue. That's how it's supposed to work.
Do you have a question that needs answering?
Yes. When I open LRCC the Import button is greyed out and it seems my hard drive in Navigator is linked to the web and won't let me choose a folder from My Pictures. Clicking Import Photos and Videos from File gives a dialogue box but this only lets me import every photo from every folder in My Pictures. There's something wrong that I don't know how to fix. Can you give me a lead please?
After you select a source folder in the Import dialogue, a checkbox appears asking if you want to Include Subfolders. Uncheck the box.
There are lots of free tutorials on the web. Here's one:
You can Google for more. Try "getting started with lightroom" as a search.