I keep kits in folders by Store, date downloaded, designer, kit name. Everything is backed up every 3 months on to a hard drive as well as a USB. Keeping backups in 2 places is a safety protocol hubby insists on.
I keep a separate file for every kit within a designer's folder and one specifically for the Previews where I add the designers' TOU. I have a folder for every designer from Oscraps, Mini O's, Collaborations, 52 Inspirations, Alphas and masks. That makes it much easier to quickly scan for just the right kit to use. The folders are in Alphabetical order.
Within each designer folder is a word document containing the store info for the designer, I have a list of kits by year/date downloaded, title of the kit and store links. (If the designer has kits from various stores, I have a separate word doc for that info but only if the kit is not at Oscraps.) example:
2021-02 Simplette Designs:
Ma Declaration https://www.oscraps.com/shop/Ma-Declaration-full-kit-by-Simplette.html
The other thing I do for efficiency, is to make a separate folder for scrap tools like masks,Alphas, brushes etc. I always start with the date. I just add the date, designer,name of the kit and tuck the preview inside. I have all designers in this each file.Then I can pop over to my designer file and locate it quickly.
example: 20-05 florju_Carnet De Voyage_Mask
That way I can scan the previews and know exactly where to find the file I want by scanning the date within the designer's folder. I do it that way because I find that if I load too many things in PSE it starts to run slowly.