Nigeriens is the answer. The inhabitants of Nigeria are nigerians.
Inhabitants of the Upper Nile are called Nilots
Different in different countries. In the UK, a city is always big, probably several hundreds of thousands of inhabitants. In the USA, some places called citieshave just a few hundred inhabitants.
They are often called Edinburgers.
Malagasy
Indios
The lucoes
Vikings.
Dominicans
The original inhabitants in the area now called Kentucky, were Native American Tribes.
The first human inhabitants in New Zealand were the Maori.
An urban region with more than 10 million inhabitants is called a megacity.