It's not. The LLM itself only calculates the probabilities of the next token. Assuming no race conditions in the implementation, this is completely deterministic. The popular LLM inference engine llama.cpp is deterministic. It's the job of the sampler to actually select a token using those probabilities. It can introduce pseudo-randomness if configured to, and in most cases it is configured that way, but there's no requirement to do so, e.g. it could instead always pick the most probable token.