shop girl vs girl

shop girl and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
shop girl	Yes	No	No	No
girl	Yes	No	No	No

As nouns, girl is a hypernym of shop girl; that is, girl is a word with a broader meaning than shop girl:

Other hypernyms of shop girl include fille, miss, missy, young lady, young woman, clerk, salesclerk, shop assistant, shop clerk.

shop girl (noun)	girl (noun)
a young female shop assistant	a young female
	a girl or young woman with whom a man is romantically involved
	a youthful female person
	a friendly informal reference to a grown woman
	a female human offspring