How does big statal project get financed?

And in particular, is it true that funds given to a project are funds removed from another project?
When and when is it not the case? In which measure the budget required is relevant to the country and what measure to actually care about?
Extra question: EU members have a debt cealing of 3%: how that change things?

I was reflecting on the fact that well placed investments can mobilitate resources that would otherwise be idle but can someone give me more nuance?