First, we should acknowledge that there is an entire subfield of political science that is devoted to studying the process of making policy decisions. Therefore, we cannot possibly give a comprehensive answer here.
The final say on matters of policy in the United States falls to Congress. It is Congress that votes for or against proposed laws. Thus, you can say that policy decisions are made by a vote of Congress.
But this is not a sufficient answer. We must at least ask what sorts of factors affect the way that members of Congress vote. Such factors might include public opinion, the efforts of interest groups to sway members' thinking, the influence of political parties, and more. All of these factors clearly have some impact on how Congress votes.
Thus, we can say that policy decisions are made by Congress with some unspecified level of influence on the part of the factors mentioned above.