Here is the Webster's Dictionary of Synonyms (1942) entry for the three words (plus citizen): Inhabitant, denizen, resident, citizen are here compared as meaning one whose home or dwelling place is in a definite location. Of these terms inhabitant applies regularly in nonfigurative use to animals as well as persons, and only denizen applies also to plants and sometimes even to words ...
Why is citizen used to describe an inhabitant of a country when the word is derived from the Latin for city (civitas) and originally meant a city dweller? Wouldn’t the nouns derived from ‘country...
etymology - Why is the inhabitant of a country called a “citizen ...
A citizen of the United States is a legal resident who has been processed by the government as being a member of the United States. A denizen of the United States is simply someone that lives there.
28 There is a suffix that is written only as -ize in American English and often -ise in British English (but not always, as ShreevatsaR points out in the comments). This suffix attaches to a large number of words, thus the s/z alternation shows up in a large number of words. Citizen does not have the -ize/-ise suffix.
By analogy with U.S. citizen, you think you can say China citizen, but Chinese citizen blocks it. U.S. citizen is different either because it predates American citizen or it means something different. e.g., it's shorthand for the legal term "citizen of the United States" (see below).
We say "U.S. citizen", but why can't we say "China citizen"? Or can we?