Amazon now lets you drop in on all your Echo speakers at the same time. Here's how - CNET

The update lets you broadcast your voice on every Echo speaker in your home, which has some big benefits.

source https://www.cnet.com/how-to/amazon-now-lets-you-drop-in-on-all-your-echo-speakers-at-the-same-time-heres-how/#ftag=CADa08b18a