More or less but there are occasions where they are not interchangeable. For example I would never say “There is a mysterious item in space” using “thing” is more appropriate here. Also, I would never place an order like this “I need to order thing number 56.” Item is a specific object here. Anyway this link will give you a side by side comparison: https://wikidiff.com/item/thing

I would say so, yes. They're what's called synonyms.