explain how does an issue become a policy and when does it become a policy
An issue is a subject about which people disagree. For example, there is a public issue over or not people should be allowed to drink at age 18 or whether it should remain 21.
Policymakers - usually public officials like a City Council will also have members for and against lowering the drinking age.
When a majority - or in some cases - 2/3 favor one side of the issue or the other, they can make a rule or policy favoring its opinion.
Also, the executive branch of government - Presidenst, Governors, Mayors etc - also makes policy. If the executive branch favors one side of an issue over another, it can make a rule like people must be 18 to drink; or if the other side wins, you must be 21 to drink.
So, an issue usually begins with people disagreeing about something.
A policy can arise when public officials take sides on an issue. Does that help