People are sometimes identified by their country of origin. The question is what you call them. In many languages, the native noun has a predicable form based on the name of the country. However, due to the wide variety of endings on country names, it is sometimes difficult to predict how to name the person.
In English, if a country ends in a or o, the person generally ends in an. People from Liberia and Morocco are called Liberians and Moroccans. In some cases, the suffix ian is used, as for Argentineans and Canadians. Ese is occasionally used as in having dinner with a Chinese and a Portuguese.
Some nationality nouns use the country as a base form and take off some letters, as in Turks, Germans, and Greeks. There are suffixes that are used in rare cases. Ard is only found in Spaniards, while “i’ is used for Israelis, Iraqis, and Bengalis. People from Latvia and Finland are Letts and Finns, respectively. Some natives don’t even have a special form, using the country name as an adjective. People from France, Ireland, and Britain are known as Frenchmen (or women), Irishmen, and Englishmen, although the latter may be called Brits. Then, there are the Dutch, who come from the Netherlands.
Aside from the official name of the nationality, there are nicknames, some of them derogatory and some not. Canadians are sometimes called Canucks (I don’t know whether they like it or not) whereas an older term for Brits is Limeys, since they were the first to have the sailors eat lime juice to avoid scurvy. To the best of my knowledge, Australians have no problems being Aussies, although New Zealanders hate being called such since they are definitely not that.
If you don’t know what you are, check the dictionary. Personally, I am an Israeli, American, and Frenchman with a little bit Iraqi from my previous marriage and can be found under the term ethnic salad.