Just what are the requirements of being an "American" as opposed to just being a resident of the United States?
No other qualifications exist, although there may be consequences that require change from the present and inherited practices of individuals and groups. There are consequences of being or becoming American.
The second requirement may need some clarification because misinformation over the last hundred years has confused people. Many believe that the United States is a democracy, where the vote of the people determines both government and social rules. Unfortunately, this apparently good idea is totalitarian, that is, it has no limitations. In a democracy, the majority dictates to the minorities. And, in reality, a small group controls the levers of power and dictates to the majority.
Rather, by design, the United States Federal Government is a constitutionally limited confederation of Sovereign States. And the Sovereign States are constitutionally limited republics. The Federal Government is just a confederation for the limited purposes of joint defense and common international relations. Within the Federal and State governments power is separated into different hands to balance the avarice of individuals and groups. This is a deliberate design to enhance control by The People and prevent the common abuses of democracy. The Constitution does not allow and hence forbids the Federal Government from dictating to the People. The goal is freedom for individuals and by extension for groups. This is an entirely new form in the history of the world.