Would be nice to know which ‘multi room audio’ devices you have - are they from B&O (means NL compatible)?
You won’t be able to turn on the BV11 automatically, if you have a Chromecast device (or an Airplay device for that matter) - you always will have to use the remote to activate the input, you have set up, before you can listen.
If you have a B&O/NL device with support for Spotify Connect, you can Join in (using the remote or the B&O app) to the listening session, once you have started the playback on the B&O/NL device.
If you use Chromecasting and you have a Chromecast device connected to the BV11, you will have to make groups in the Google Home app in order to have a multiroom experience (and as mentioned, you must turn on the BV11 manually).
Same goes for Airplay - if you have e.g an ATV connected to the BV11, you must tick the device in the AP menu on the phone/tablet....and turn on the BV11 manually.
MM