It sounds like you've purchased a Chromecast with Google TV. You don't have to use the Google Assistant features if you don't want to. You can very much still cast from apps on your phone, or cast a tab in Chrome from your computer to your Chromecast, and don't need to use the Google TV apps if you don't want to.
If you decide to return it, you should be aware Google no longer sells the older Chromecast models directly, so you'd need to buy one from a reputable third party retailer.
