Can anyone explain the loot/item system?

I do not own the games but have been eyeing them for awhile. I saw that there was a Steam sale recently.

I’ve seen screenshots of items with many attributes/qualities and am kind of curious how that comes into play during a runthrough. Do you pick up all valuables , craft them, alter them to your liking, are there such things as ‘runs’ where you head out to improve your loot, etc