Hi! you'll find some additional information on the term "American" and demonyms for citizens of the USA in these threads:
When did "American" become the demonym for citizens in the United States?
How can one explain the issue about the term "America"?
What is the origin of the word Estadounidense in Spanish? Further, what were the demonyms of the Las Provincias Unidas de Centroamerica y las Provincias Unidas de Sudamérica? Were they both provinciasunidenses, or were they called centroamericanos and sudamericanos?