Learn English – name for adjectives like “beautiful”, “incredible”, “amazing”, etc.

Adjectives are the prototypical modifiers of Noun Phrases. Generally speaking we kind of need a specific reason to use a Noun to modify another Noun when an adjective is available. There are many such reasons, but a common one is when there is simply no adjectival alternative - in other words when it simply doesn't exist. Because there is no reason not to use the adjective Chinese when modifying nouns, we do! We should note that by and large there's no difference between modifying the Noun citizen or any other Noun used in formal English:

• Chinese citizens, officials, economics, food regulations

When the name of the country is a Noun which has no readily accepted Adjective as a counterpart, then, as Jlovegren says, we use that Noun as a Modifier in the Noun Phrase when we need to modify another Noun:

• the Myanmar people
• Benin culture

When it comes to countries whose names are already compounds, then it depends on the head word in the Noun Phrase that constitutes the name. If that head word has no Adjective counterpart we will use the name as a Noun modifier. In the United Arab Emirates for example, Emirates is the head Noun in the Noun Phrase. We have no Adjective for Emirates (Emiratian or anything like that), so we have to use the whole name as a Noun modifier. The same goes for the United States. There is no Adjective for States. So we have:

• United States citizens
• the United Arab Emirates economy

This can be contrasted with, for example South Korea. Korea is the head of the Noun Phrase here. We have an adjective relating to Korea, namely Korean, so the following is perfectly fine:

• the South Korean food industry

Similarly we have an adjective corresponding to Britain in Great Britain, so we can talk, for example, of:

• the (Great) British public

Other examples are:

• the Saint Lucian president, the South African Government, the Saudi Arabian peninsula

... and so forth.

This won't work of course if the head noun is just a generic word like Republic. In fact it won't work if there is any phrasal genitive, X of Y construction such as we often find with Republic. Consider:

• a United States of American citizen (wrong)
• a People's Republic of Chinese statesman (wrong)
• a Dominican Republican citizen (wrong)

Instead, when such compounds are used we find:

• United States of America citizens
• Republic of China citizens
• Dominican Republic citizens

Very often we have more than one name for a country. Often one is longer and more formal than the other. If there is an adjective for the shorter name we will use that as a modifier accordingly:

• Vatican officials

However if we use the compound name, then as described above it will depend on the head Noun in the Noun Phrase. So in instances where the head Noun has no adjectival counterpart we will find examples such as:

• Vatican City State officials

Similarly, countries primarily known by their abbreviations will also have the abbreviations used as modifiers in Noun Phrases, because there are no adjectives (or at least well-known ones) relating to the names of letters! So we have the British government, but also:

• the UK, US, and UEA governments

This goes for abbreviations in general, so we commonly find phrases like:

• WWF supporters, IBM contractors, FIFA officials, IMF spokesmen

Hope this is helpful!

Along with numerals, and special-purpose words like some, any, much, more, every, and all, they're Quantifiers. Quantifiers are a kind of determiner and occur in many constructions with other determiners, like articles: e.g, two dozen or more than a score. If you want a special term for the words that describe numbers like these, you could call them lexical quantifiers, but then you'd have to explain what you meant because it isn't standard. — John Lawler

Quantifier has a nice entry in Lexico.

1.1 Grammar A determiner or pronoun indicative of quantity (e.g. all, both).