I didn't understand the rules completely. The jam says you can use code from old projects, but then says premade assets aren't allowed? I'm kind of confused. Are we allowed to use Store assets such as FinalIK?
Yes! FinalIK and any other Unity Asset Store asset is totally fine! You can also use pre-made tools that you have developed personally, but aren't on the asset store. The spirit of the rule is just don't start ahead of time.