Can we use assets such as character controllers, AI, etc that control game logic? And also, can we use models, audio, or code fragments from previous games?
The short answer is, "Yes, it is fine to use previously created assets". The longer answer is "be reasonable about it". If the pre made assets you use make up the entirety or a major part of your jam submission, that is not ok. If you have a script or two that are pretty standard or will help you with a feature or two, that is fine. There will also be a place for you to write a note about this when you submit your jam entry.
The rules say "
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