Bitcoin is completely unrelated to how casinos generate their rolls - unless you're talking about a site like Chain-bet, in which case they simply take the last digit of each bitcoin block hash (which is random) and take that to determine outcomes.

I suggest you take a look at how provably fair systems work.

From the gambler's perspective, you don't really need to know the technicalities. Just know that you are able to verify each roll using your client and server seeds after the event to make sure that the casino is not cheating you.