Are you able to create routines under Google assistant based off virtual switches or switches turning off or on?
Sadly not. Routines on GA are voice activated only at this time.
Nothing elegant, but as an example going from GAR to HE...
Going in the other direction is dubious, again but...
My Hub Max sits across the room from my Sonos Move, so when no one is home, I had fun letting Move say the magic word commands to my hub.
Ugly, I know, but poking away. lol
Fascinating leads include:
Puppet - A .NET Core framework for automating Hubitat Elevation
.NET Core Guide
@dman2306 GitHub - dcmeglio/hubitat-myq: Integrate SmartThings with MyQ
Yeah, I've used my Sonos to chat up one of my GH devices before. It works surprisingly well. So from Hubitat TTS on Sonos it's possible to get Google to do whatever you want that way