By default they are matrix multiplications. Temperature is added in as forced PRNG because testing found that correlated with better outputs.

Given the same prompts and the same weights, one can get the same answer each time.

In practice, there are a number of optimizations that makes the results dependent upon thing we give up control of to increase performance, meaning the results end up being effectively non-deterministic. But, if you are willing to run it in a slower mode so we don't do some steps out of order to speed things up and don't batch results (or if you consider the determinism of a given batch of requests rather than individual requests), then the same input gets the same output.