Any time there's an internet-connected microphone nearby that doesn't have a physical cord you can remove there's a risk that it's recording audio and transmitting it someplace. Is that always true? No. Is Android "always listening"? If you have the options to call Assistant hands-free enabled, it absolutely is. Can assistant send audio to the cloud? Sure, but it's supposed to only be for processing when you call assistant for something.
iOS isn't immune to this. A co-worker was just telling me today that he and his wife were having a discussion about a product, her iPhone was next her with the screen off. She then picks up her phone, opens Facebook and BOOM - ad for the exact product they were talking about. Yes, HE uses Android but he doesn't have a Facebook account and it was HER iPhone that this was seen on...