The gross loss is driven by $920M in losses on firm purchase commitments. As I understand it, Rivian has entered into contracts with suppliers to buy some components needed to manufacture its vehicles at fixed prices over some future period of time, likely several years. If the market prices of those components decrease, the value of those contracts also decreases. In some sense this isn't a "real" loss - Rivian will still be buying the same components, at the same prices, on the same dates that it expected to at the start of the quarter. But the market prices of the components went down, which creates a paper loss, which they have to mark to market.
That also creates a timing mismatch - Rivian's revenue in Q4 was $663M and its cost of revenue was $1663M, but most of the $1663M is associated with vehicles that they haven't delivered or even manufactured yet, so they'll recognize that revenue in future quarters. I don't know what it actually costs them to manufacture one vehicle, but I bet it's a lot less than 2.5x the revenue they get from that vehicle.
Leaving aside the accounting and addressing your real question: From a quick search it looks like Rivian has raised a total of $23B of capital between VC rounds and its IPO, and it has $13B of current assets (mostly cash + inventory) as of 12/31, so that gives it a fairly long runway even at its current burn rate. But its burn rate - loosely, revenue minus expenses - is expected to slow over time, assuming that (1) its revenue grows faster than expenses (it's expecting deliveries to ~double this year, which should increase revenue at a similar rate) and (2) its unit economics work out, i.e., its "true" cost of revenue is less than its revenue.