How does fast food restaurants make grilled chicken In most of the fast food restaurants which do grilled chicken, when they get an order, they take a chicken which looks like it is pre-cooked as it is yellowish in colour, and put it on the grill

This is the case with restaurants like Nandos etc. During the grilling they apply the sauces and then serve

How do they precook the chicken? Do they marinate it and use an oven to heat it?
If you are wondering explicitly about Nandos, it is marinated for 24 hours before its cooked. It is then cooked before its ordered: 


  [The chickens] are cooked in a special oven for an average cycle of 30 minutes,
  reaching a temperature of at least 75C, then kept in a warming drawer
  at a temperature of 63C.


Source

When an order is placed, they take the Chicken and finish it on the grill.