I bought a few minor things and thought they would be in my "collections" or something- in case I switch computers or whatever & need to download them again. DriveThruRpg does this- you have a library of purchases. Anyone?

"Collecions" are the main sets which you would have purchased (so far).
"Kitbashes" are just that - Kitbashes. These may been ones you specifically purchased because you wanted that particular work - and or, the ones given to you free because you already own the original set(s) from which they are derived. Kitbashes are fan-made variants from the main sets - which cannot be sold because they derive from the original artist's efforts, but WWG distributes these free to whomever already has the original set.
Then there are the "Freebies" - 'nuff said.

So, you have to look in all your sub-folders.

There are several sets which have spurred much enthusiasm and attracted many additions - titles such as Mayhem MegaMall, for instance. [Thank you, Jaynz!] So if there is an area/style/genre that you have a great interest in, check out the main set in the catalog. There will be a listing of all the kitbashes on the right side of the screen which you will receive if you purchase that main set.

I hope this helps.