You've been able to listen to the radio via the Google Assistant for quite some time already — in fact, Home devices have supported TuneIn since launch. The company is now adding an option to integrate paid radio services such as SiriusXM to the Assistant by rolling out a dedicated radio section in the Assistant's Services tab.
Once the new feature has rolled out to you, you can sign into SiriusXM and use it on the Assistant. To add your account, head to the Assistant setting's Services section, where you should find Radio, and then choose SiriusXM, the only radio service available for now. Google will walk you through a privacy disclaimer and offer you to sign up or connect existing credentials. After you've set everything up, you'll be able to play exclusive SiriusXM stations by saying "Ok Google, play xyz on SiriusXM."
For now, subscription radio services are only available in the US and Canada, and it's only SiriusXM. We might see more radio services utilizing this feature in the future, and it will probably expand to more countries at some point.