While I agree that simulation theory pushes the question up a level, that is not the case for the God of classical theism.
A necessary thing, by definition, is its own explanation. "I am what I am" etc.
From there, classical theists attempt to connect that necessary thing to what you'd commonly understand as God.
The answer to Big Bang is that before it, there was no time itself, so there's no notion of "before" (or if you wish, it's an error in the question itself that assumes there was "before").