You can create an automation that runs when a defined Focus mode is turned on and/or off. I've just tested it, and I can have an automation which turns lights on when the Focus mode is activated, and off when de-activated. I don't have a use for this, but I'm tempted to invent one just because I can.Not with the built-in Focus mode tools. I'm not sure if there's some way you could do a Shortcut or something, but there's no mechanism for activating smart home products from the Focus automations.
You may be able to have an automation that triggered when opening the Phone or FaceTime app, for example, but it would trigger even if you just opened it to check the recent calls or something. Plus it wouldn't get triggered if someone called you as that, for some reason, doesn't count as opening the Phone/FT app.