Confusing LR Classic Catalog Hierarchy

I've been using LR for many years and don't really understand the catalog system.
I am using LR Classic V12.5. Since I'm about to upgrade my PC to a new drive, I want to understand my catalog system and make it more streamlined.

On my main PC drive, I have a folder in my Pictures folder named 'Lightroom Folder' I haven't cleaned out my past catalogs so I know there are a ton of very old LR files I could delete. But which ones?
My current LR is using a catalog named Catalog-2-2-V12. Inside the Lightroom folder there are 3 folders and one white icon for 'V12'.

Folder 1: Catalog-V12 Helper.lrdata
Folder 2: Catalog-V12 Preview.lrdata
Folder 3: Catalog-V12.lrdata-data

White icon named Catalog V12.

See photo.

How do these all work together and what do they each do?
If I want to clean up my Lightroom folder, how can I delete what I don't need and keep the most recent backups? Maybe the last 3 or 4.

Any help on how to maintain a clean and tight Catalog system on my PC will be appreciated.

Thanks, Bob

The catalog is just like the card catalog in a library: it points you to images where they reside. 
The three files you list all work together and are 'linked' to provide differing functions; Preivew.Lrdata contains the previews. LRdata is the catalog itself. You could delete the preview doc and regenerate them if you had to. Not suggesting you do, only that it is linked and 'built' from the LRdata catalog. Helper.LRdata is a cached data to help performance; again, leave it alone as it is part of the catalog files that work together.  V12 indicates the catalog was updated in version 12 from an older catalog. And there is no reason you can't delete a group of older catalogs, especially backups** You can place your catalog files together anywhere you wish. I have mine on an external drive dedicated to just the catalog and all my images. IOW, none of this has to be in the Pictures folder. 
** LR's backup schema is lame and only backs up the catalog files, not your images. I never use it! I back up everything using a dedicated backup application. But you can delete older LR backups if you wish. 

Not sure about all those folders, I only have one catalog (no need for more), and the one folder structure appears below. The only minor difference would be the Settings folder which is optional with the Store Presets with Catalog option, which I greatly prefer:

Just a note: a couple of days ago I noticed that my current 'all photos' catalog only showed my photo folders for November. I eventually had to revert to the previous version of the catalog and re-import my 'November' photos. So keeping the last 3 or 4 versions is a good idea, just in case. This is the first time this happened to me in some 8 years of using LR. 

Mike