I've seen the Big Bang referred to in two ways: (1) the sudden expansion of our universe very early after it began (inflation) or (2) the the very moment that the universe came into being. In the first case, time already existed. In the second case, the Big Bang was the very first moment of time.
By "finite" I mean a finite amount of time has elapsed since the Big Bang.
They are, in fact, only analogies because we can't perceive four-dimensional shapes. For a more proper understanding of the racetrack analogy, we can imagine that the universe is an infinitely-thin ring and any one-dimensional being living in that ring can only move in two directions: forward and backward. Those are the only two directions that they can perceive or even understand. In a sense, they are living in the edge of their own ring universe. The only directions they can move in will eventually bring them back to their starting point (if we ignore that others will invariably get in their way, since you can't go "around" things in such a universe).
The sphere (or balloon) analogy is the more common one seen in cosmology. The analogy is made by thinking of two-dimensional beings that live embedded in the surface of an expanding balloon, which represents their own two-dimensional space. They can move up, down, left and right and perceive these directions but they can't move "in" or "out" of the balloon's surface because their space is only two-dimensional, not three-dimensional like ours. Like with the ring analogy, they are sort of living inside the edge of their own universe.
Nothing cannot exist, of course, as that would be a self-contradiction. That doesn't mean that time goes back forever, though. Nothing can come before time without also being an oxymoron, so questions like "what was there 25 seconds before the beginning of time?" are as nonsensical as "what does a piece of aluminum smaller than an aluminum atom look like"? This will be true regardless of how the universe came into being.