The book and movie do a pretty solid job about explaining what CDOs, derivatives, etc. are.
What the book and movie don't do (unless I'm forgetting something) is explain how anything involved with the 2008 crisis was illegal - the sole exception being that CDSs are very arguably insurance contracts and should be regulated by insurance laws.
The Big Short made it sound like credit rating agencies engaged in intentional negligence, which doesn't strike me as something "too abstract to prosecute" (or whatever the excuse is supposed to be here).
Negligence in general isn't a crime - it does become a crime if gross negligence directly causes death or major injury, but as far as I know, you can't prosecute someone if their negligence merely costs someone some amount of money, no matter how much. It may cause liability for that amount of money, but that's not a crime that the government can prosecute and threaten with jail time, that's a civil case between them and whoever suffered and may sue them for some money.
