LLM are just software + data and can be made deterministic, in the same way a pseudo random number generator can be made deterministic by using the same seed. For an LLM, you typically set temperature to 0, or set the random seed to the same value, run it on the same hardware (or emulation) or otherwise ensure the (floating point) calculations get the exact same results. I think that's it. In reality, yes it's not that easy, but it's possible.

Unfortunately because floating point addition isn’t always associative, and because GPUs don’t always perform calculations in the same order you won’t always get the same result even with a temperature of zero.