Assuming not a modulating thermostat, just simple on/off, then thermostat works the zone valve, and zone valve works the boiler.
The idea is many fold, it insures the zone valve is open before the boiler fires up, it used the zone valve as a relay so many thermostats can control the same central heating boiler, it removed high loads from the thermostat.
However there are exceptions, one problem with heating is the time delay between heat leaving boiler, and getting into the room, it causes hysteresis i.e the system over shoots, today we go to great lengths to reduce this hysteresis.
The standard method is to keep the mass of heated bits low, so with a fan heater, switch on and in seconds we are warm, switch off and in seconds we feel the cold again, with a water heated fan heater we have two options to reduce this, alter water temperature (modulation) or change fan speed. Again with a storage heater the reverse is true, switch on and it takes ages to warm up, and switch off and ages to cool down, again fan speed can help.
So with under floor heating your limited to 29°C so this can be controlled by the zone valve, unlike normal zone valves they often don't switch on/off but regulate the flow, as a result they will not have micro switches which control the boiler, they often have complex systems to regulate the floor temperature.
So the thermostats talk to each other often using IFTTT which means "If this than that" what this means is some clever person writes a program often stored on the internet which talks to the devices, so for example it may access weather reports for the area and if a cold snap is forecast it heats up floor, but if likely to get warm as day progresses it will only heat the air in the room to stop this hysteresis.
We get electronic thermostatic radiator valves (eTRV) which talk to wall thermostats which in turn talk to boilers, So MiHome TRV heads and Nest thermostats and the Heatmiser can all connect using follow commands and IFTTT. It gets really complex.
Most seem to go for some thing like EvoHome as far better if you don't rely on an internet connection, also then we also get modulating boilers with OpenTherm so instead of turning boiler on/off you turn the flame up and down again reducing hysteresis.
So before you install the first pipe or radiator one plans how it will all talk to each other, not easy as new apps for IFTTT are coming up all the time.
So the question is what were your plans? If you detail your plans and then say I am stuck with this section than maybe some one can help, but it has to be part of a larger plan.
My son fitted underfloor heating to get rid of excess hot water from the side boiler on the solid fuel kitchen range. Not to heat house, although clearly it does, but to store heat.
So please explain your plan.
