shop girl vs shop assistant

shop girl and shop assistant both are nouns.

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

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

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

shop girl (noun)	shop assistant (noun)
a young female shop assistant	a salesperson in a store