https://www.cnbc.com/2019/04/13/how-to-delete-your-voice-recordings-from-google-assistant.html

Like Amazon, Google stores and collects the things you say to its assistant on your phone, computer or smart speaker, like the Google Home.

Google's privacy page says it does this to "help you get better results using your voice," and that it only does this after you say "OK Google" to learn the sound of your voice and how you speak certain words and phrases.