After you complete flight school, you obtain a CPL/ATPL, and the plane that you flew in flight school would be most likely a twin engine. After you complete flight school, and say get to work with MAS or AirAsia, you can't simply jump into the right hand seat and fly the plane.
You need to complete a type rating before flying that specific aircraft. Not to mention you have to first do a jet conversion before a type rating. So, if you work with MAS, you'd have to complete a B734/B738 type rating to be certified, to be able to fly the aircraft. In the case of AirAsia, you need a A320 type rating to fly that aircraft.
In short, type rating is an additional requirement on top of your CPL/ATPL license to operate a specific commercial aircraft.
dinos9 wrote:Then the requirement to get the type rating is by paying upfront payment of 30k USD? Or it's actually involved any addition trainings?
Typically, type rating can be done at the airlines you are hired from. But if you have money to complete a type rating on your own, you can get any type rating you choose. For example, after completing your CPL/ATPL, you could go ahead and get a A380 type rating in Toulouse if you want. Then you could apply to airlines as a direct entry pilot to fly their A380's. That said, I don't know anyone that did such a thing. Also note that airlines have their own requirements if they would let you fly their A380's without any prior experience on twin-jet operations and no PIC hours.
Type rating certification involves hours on a flight simulator and most certainly ground school, to familiarise yourself with the aircraft (i.e. aircraft specific operations, electrics/avionics of that specific aircraft). It's like your driving license, you need to get your L (in this case, learn the aircraft systems) and then your P (simulator training).