TRV’s do indeed send a heat demand to the device connected to the boiler or to a smart thermostat.
When you add the TRV’s and thermostats to each room you specify which device is the controller for that room. So if all you have in a room is a TRV then that will be the controller and it will send heat demands directly to the device attached to the boiler.
For example my cloakroom and en-suite only have TRV’s so they request heat from the boiler when needed. My lounge has two radiators so I have added a smart thermostat to that room. That smart thermostat will then request heat from the boiler and set the percentage opening of each TRV.
