Some cameras, like the Google Nest offering, but a quick google tells me some third party cameras as well support viewing the camera stream in the Google Home app in addition to being able to cast it to a Chromecast.
Unfortunately camera streams exposed via Home Assistant to Google Assistant for whatever reason do not support this. I’ve tried setting the stream config to use hls and some other options which didn’t seem to help so I’m not sure what the exact feature is that’s missing from making this work.