How do they use HVAC system in heating buildings?

An air conditioner cools and dehumidifies the air as is passes over a cold coil surface.
The indoor coil is an air-to-liquid heat exchanger with rows of tubes that pass the liquid through the coil.
Finned surfaces connected to these tubes increase the overall surface area of the cold surface thereby increasing the heat transfer characteristics between the air passing over the coil and liquid passing through the coil.
The type of liquid used depends on the system selected.
Direct-expansion (DX) equipment uses refrigerant as the liquid medium.
Chilled-water (CW) can also be used as a liquid medium.
When the required temperature of a chilled water system is near the freezing point of water, freeze protection is added in the form of glycols or salts.
Regardless of the liquid medium used, the liquid is delivered to the cooling coil at a cold temperature.

Usually a Heating coil is added within the Fan coil Unit or Air Handling Unit so as to shut of cooling and supply warm air .
A separate Heating Arrangement can also be provided depending on requirement.

Modern Building expecially in Hospitals and schools provide the special type FCU 's have four inlet and four output two of them conected the CHW supply and Return and another is hot water providing by Boilers out put and another type is by heating coil by Electrical conections controlled by the humidifier thermostate .

HVAC System is a simultaneous process of controlling Temperature, Humidity, Air movement and Air filtration (Maintain Purity of the Air).
By controlling of the above factors Air conditioning takes place, in any buildings.

There are two type of heating sytem used in HVAC.1.
Electric heater2.
Hot water system There are different type of control system are available for temperature control.
Basically electrical heaters are control by the number of stage depend on the Kilowatt of heater and hot water system control by motorised valve acctuater.

as mentioned above, it depends on the HVAC system type installed in the building, if it is chilled water system, heating can be achieved by hot water supplied from the boiler to the heating coil of the AHU, or by using electric heaters installed in the AHU for single zone or in the VAV for separate zones.
In DX-systems, a reversing valve can achieve the heating by reversing the refrigeration cycle, it also can be achieved by electric heaters in the AHU or VAVs in multizones areas.
one common process for heating inside buildings is through the heat recovery unit, where heat rejected from areas to be cooled will go heat exchange process with the air of the areas needed to be hot or warm, through energy wheels in AHUs or heat recovery units in DX-systems especially in VRV applications.

HVAC is the short-form of : Heating, Ventilation & Air-conditioning.
Fundamentally, any air-conditioning system is a a heat-pump which pumps the heat-load of one closed space to the other side, which usually is an open space.
In normal case, and common understanding - AC works against the nature's force, meaning it throws heat from a comparatively low-temperature space to the higher temperature zone, thereby disposing off the human/elect.
instruments heat/external heat-leaking in - back to the external space.
By reversing the beneficiary, effective zone of HVAC's working outlet, which normally is the colder side, the resultant zone can be heated as well.
In normal practice the fan-coil unit, called as FCU, has a fan blowing air over a coil-unit, having a continuous internal circulation of refrigerant/cold-water.
This air blown over the COOL media (FCU) gives the cooling effect.
Similarly, to give heating effect the fan blows air across a coil which either has the heating effect using electricity or some other source of heat.
For industrial or large-commercial application, it can even have some heating chemical (thermic fluid) circulating through the coil.