I met Muslim here in Germany and other countries including my co-worker(Turkish, Arabic and African) - they consider themselves as Muslim, some of them prefer to consider themselves as an Turkish, Arab or African, not Muslim)... It relate to their different culture background. One of my Turkish co-worker said that he consider himself as a Muslim instead of Turkish whom he don't worship Islamic belief... I have been travelled to Islamic countries before and consider Islamic countries as Muslim world. I respect their different customs and culture.
This is definition of my interpretation view.
Some Turks, Arabs, and Africans are Muslim, some are not. Some Turks, Arabs, and Africans are Christian, some are not.
If a person is a Muslim, that means his religion is Islam.
If a person is a Christian, that means his religion is Christianity.
If a person is a Jew, that means his religion is Judaism.
If a person is a Buddhist, that means his religion is Buddhism.
Those are religious groups.
If a person is a Turk, that means he is from Turkey.
If a person is an Arab, that means he is from an Arab country.
If a person is a German, that means he is from Germany.
If a person is an American, that means he is from the United States.
If a person is an African, that means he is from the continent of Africa.
Those are not religious groups.
Muslim people follow the Islamic religion.
Christian people follow the Christian religion.
Some Turks are Muslim, some Turks are Christian.
A person's nationality is the place where they were born or currently live as a citizen. It's not a religion. There is no country called "Islam" or "Muslim."
I hope this helps to make it clear.