Your voice is always sent to Apple to process it since Siri doesn't work locally; what we are wondering is whether that voice recording is permanently stored or destroyed as soon as it's processed.
They have a good track record of respecting user privacy.
Yes, let's try again, how do you know you can trust anybody to do anything?
Do you hold a gun to their head? Do you trust the courts to uphold written agreements/contracts that dictate the other party's behavior? Do you observe their past behavior and use that to guess future behavior?
In 99% of our daily actions, we use the last option, and right now is no exception.