As far as I know, you can’t tell Siri which device to listen to, Siri uses AI to do that. The best you can do is put your iPhone and iPad face down on a table if you don’t want them to respond to Siri. Otherwise Siri will think those devices are active and add them into the AI mix. When I want the iPhone to respond to Siri and not the HomePod I hold it up to my face. Sometimes the iPhone responds, sometimes it only flashes the Siri screen briefly and passes the command off to,the HomePod.