If you want to improve your voice recognition, then you need to have a ground truth. To continuously improve, you must validate, isolate and update the corner cases.
This means listening to the failed speech to text events, ones with low confidence, and a sampling of high confidence events.
This is literally what google voice was for, it was a way of google getting lots and lots of samples of people talking using a phone's microphone.
Almost certainly google/apple/amazon will be sampling photos, emails anything that goes through a ML pipeline, humans will see.
Now, ofcourse apple have positioned them selves as a privacy first company, so this is a big no no. They should have re-asked for permission (I'm pretty sure they talked about this in the Terms and conditions, there was some noise about it when siri first came out. )