Theorectically, an immigrant could enter the United States at any point which was provided with immigration officers. In practice, however, they were directed to certain selected entry points. For immigrants from Europe, that would have been Boston, New York, Philadelphia, and, to a much lesser extent, Charleston and New Orleans. Far and away, the most immigrants arrived in New York. Those who landed in other ports were those whose application for admission had been processed in the home country before sailing for the United States. In 1920, that wasn't very many people, and almost all European immigrants arrived at Ellis Island, which was, sort of, in New York.
There is an island at the mouth of the Hudson River, which was enlarged for the purpose of building the immigration center there, and it is known as Ellis Island. The original, natural island lies in New York, and is treated by the city and state of New York as a part of the borough of Manhattan. The larger portion, created by landfill, lies in New Jersey, and is treated by New Jersey as a part of Jersey City. Nevertheless, it was the main point of entry for the United States and new immigrants who were allowed to enter were ferried to New York city. The immigration center at Ellis Island operated, roughly, from 1890 to 1950 (or a few years after). In 1924, an immigration quota act took effect, and immigrants were processed at United States embassies, legations and consulates in their home countries, and were free thereafter to land in whatever port the steamship company took them to.
Ellis Island was used only for refugees and displaced people after 1924. Ellis Island is now under the authority of the National Park Service.
Click here to visit the National Park Service official site for Ellis Island. You can probably find more information by doing a web search for "Ellis Island."