It replaces your standard wired doorbell and does require a compatible chime and transformer which are each sold separately.

Google Nest Hello doorbell requires transformer voltage ... Pinch the plastic clip and insert the FRONT wire into the clip on ...

FYI: The Nest Doorbell doesn’t come with a chime. You can either connect it to another Google device with a speaker like the Google Home Mini or wire it to your existing chime. The Nest Doorbell ...