Why is it that the American public tends to be ignorant on issues regarding American foreign policy? Generally speaking, in what circles are foreign policy decisions made?

One reason that many people in the United States may be unfamiliar with the issues regarding U.S. foreign policy is that, for most people living in the U.S., the implementation and impact of U.S. foreign policy lacks immediacy. The geography of the U.S. lends itself to a sense of isolationism, as the primary land mass of the country is only bordered by two foreign nations – Canada and Mexico. And even with respect to those two nations, most of the impact of U.S. foreign policy is felt in the states that border Canada and Mexico. Thus, for most of the population of the U.S., foreign policy is something that pertains to places far away, and the implications and responses to that policy have little direct impact on their day-to-day lives. It is only when issues pertaining to U.S. foreign policy directly impact the U.S. (e.g., violence by foreign nationals on U.S. soil; criminal activity coming across the U.S. border; violence perpetrated in the context of border crossings; etc.) that that people in the U.S. start paying close attention.
Another reason that many people in the United States may be unfamiliar with the issues regarding U.S. foreign policy is that they feel detached from decisions regarding that foreign policy. Some of this detachment may be attributed to voter apathy, and some of it is likely due to voters caring more about domestic issues rather than external policies, but much of it seems to come from a sense that foreign policy is outside the realm of understanding of regular people. Just as foreign policy is viewed as pertaining to people and places far away, the people making the decisions regarding foreign policy are often regarded as separate from everyday people. Many people attribute foreign policy to “the government” without ever thinking about who is making the decisions.
Those decisions, for the most part, are made by the executive branch. Under the U.S. Constitution, the chief executive has the power to negotiate with foreign countries, make treaties, appoint ambassadors and other diplomats, etc. Further, the U.S. State Department, which implements U.S. foreign policy and the secretary of which is the top diplomat in the U.S., is part of the executive branch. Congress has the power to ratify or reject treaties, and of course provides the funding for the State Department and the various U.S. embassies and consulates, and the U.S. Supreme Court can review and overturn treaties and laws pertaining to U.S. foreign policy that are not compatible with the U.S. Constitution, but most of the decisions regarding U.S. foreign policy and its implementation rests with the executive branch.