Question

how is a major policy decision taken?

Answer 1

The government is the major decision maker who issue notice known as office Memorandum for issuing some order.

but there are political parties involve in the decision that holds the government.

As the head of a state ,  A President  have major formal authorities, Without him no notices can be issued.

Also, the Prime Minister along with his Cabinet decides about decisions, because Prime Minister is the head of government.

In order to perform or exercise a function a Prime Minister must have majority support in the "Parliament". 

Answer 2

After passing of the bill from both houses of a parliament,
then it go for president consent and assent.
if president gave his assent then a major policy decisions is taken