For simple tutorials, it's not hard to "copy" their code yourself but make it unique in your way enough that it's not really stealing. The unity documentation often has example code to explain how a method or property works, and you can copy that as long as you edit it to make it relevant to your code. An experiences unity developer will probably write exactly the same code from memory anyway, so it can hardly be called "stealing"
Everything has to be made during the jam. This means you can't reuse any code or assets you made before the jam or from other sources.