From The Guardian:
Apple contractors regularly hear confidential medical information, drug deals, and recordings of couples having sex, as part of their job providing quality control, or "grading", the company's Siri voice assistant, the Guardian has learned.
Although Apple does not explicitly disclose it in its consumer-facing privacy documentation, a small proportion of Siri recordings are passed on to contractors working for the company around the world. They are tasked with grading the responses on a variety of factors, including whether the activation of the voice assistant was deliberate or accidental, whether the query was something Siri could be expected to help with and whether Siri's response was appropriate.
Apple says the data "is used to help Siri and dictation ... understand you better and recognize what you say".
But the company does not explicitly state that that work is undertaken by humans who listen to the pseudonymised recordings.