installing hooks

How are the hooks for fans and chandeliers installed in ceilings at the a building is constructed ? and also after construction. ?

The best way is to weld the hook to the reinforcement steel bar in the ceiling at known locations before the concrete is poured. In existing ceilings it is better to dig around till the steel is exposed and fix the hook preferably by welding.
Simpler methods can work for smaller weights.
https://video.about.com/interiordec/Install-a-Ceiling-Hook.htm

bioramani

The guy in the video installed a ceramic hook to hang only a flower basket. even though the idea of using an anchor was good.

Now what i want to make sure is that the hooks which are attached to the steel bars by welding, will those welds be strong enough to support heavy loads , such as ceiling fans ?

That is what I have used everywhere in my house ten years back. Since the load is directly transferred to the reinforcement there is no failure. Actually the hook is itself hooked over the reinforcement bar. There is no way it can fall down. It works even without welding.