Most of the pieces are real but they have been built into something that is not reality.
That we have bunkers? Reality.
That we are building more? Possible.
That a bunch of money has gone missing? Reality.
That it was used to build one giant underground bunker? Probability really low.
For one thing having a one giant bunker would be akin to having all your eggs in one basket. Not a good plan.
You would also have to have a way to move large numbers of people to a certain area all at the same time without causing major traffic jams. That would be a logistics nightmare.
Mind you it would not be the stupidest thing the government has ever done. But it would be up there.
The “conspiracy” claim is that there are maglev trains that connect western military bases—several with their own underground “cities” available if needed.
I do not know that it is true—but with enough funding I see no technology or personnel constraints that would prevent it.
In the event of a nuclear war that would be the best way to maintain continuity of government—with a military command.
You could not count on civilian leaders being saved in time.