Confirmed with Google Support - all Home devices (Hub, Mini, etc) within a designated Home will broadcast visitor announcement alerts simultaneously when the doorbell is pressed.
It would be great to be able to specify which Home devices would broadcast the visitor announcement alert in order to prevent potentially multiple alerts from going off at the same time. This can be especially annoying in small homes/apartments with multiple Home devices.
For instance, disabling visitor notification alerts on a Home device in a child's room to prevent them from being disturbed while napping.
