We have Dutchmen and Frenchwomen and Scotsmen and Welshwomen and Irishmen?

We have Spaniard for a Spanish person, and Italian for an Italian person though Italian is the same as the language, and Germans are German and speak German, but you can’t just say “an English” but you can just say “a Scott” but you can’t just say “an Irish”.  Americans are Americans and the official language is English, because we were a colony, and Mexicans are Mexicans and speak Spanish because they were a colony.  Brazilians are Brazilian and speak Portuguese because they were a colony.  Actually what is someone from Portugal called?  They’re not a Portuguese (are they?) they’re a Portuguese person, right? It seems like you can’t use nationalities ending in -ese as a noun without coming off as hostile.  Romans were Roman but spoke Latin?  

An Egyptian is an Egyptian and the official language is Arabic. A Greek is a Greek and the language is Greek, they’re only Grecian if they’re ancient. An Ethiopian is an Ethiopian and Amharic is the official language, but there are numerous other languages official to the various ethnic groups.  It seems like nationalities ending in -an like Ethiopian, Egyptian, Indian, Iranian, Cambodian, American and Armenian and so on can be nouns used to refer to people, and are often used for countries where the name for the people isn’t the name of the official language.  Egypt and Iran are the only countries there than don’t end in -a?  Why are people from Egypt Egyptian, and Scots people Scottish, and from Scotland, and people from New Zealand are New Zealanders and not New Zealish?

Nationalities ending in -ish like English, Irish, and Scottish can’t be nouns unless you’re referring to the language, but Scott eccentrically can, and I guess historically the Angles (the Germanic group that are why England is England) are Angles and that can be a noun.

A Turk is Turkish like a Scot is Scottish and a Swede is Swedish.    Welsh can’t be a noun, unless you’re referring to the language, same for French, and Dutch, maybe -ch and -sh endings can’t be used as nouns for the people?  A Pakistani is a Pakistani, and an Iraqi is an Iraqi, -i endings can be nouns for the people and you can say a Pakistani man, or an Iraqi woman?  A New Zealander is a New Zealander and it’s odd to say a New Zealander man, but one usually says a Kiwi and would say a Kiwi man, or a man from New Zealand, a New Yorker is a New Yorker (I’m having trouble thinking of another country with an -er ending) and you wouldn’t say a New Yorker woman, you’d say a woman from New York.  Someone from the Philippines is a Filipino/Filipina/Filipinx.

HOW DOES THIS WORK?  Like why are suffixes the suffixes they are?  Are there even rules?