"The apples are the girls'."
Translation:Äpplena är flickornas.
I'm not a native speaker, so forgive me if this is wrong, but I think "flicka" translates into "girl" whereas "tjej" is more along the lines of "young woman" or the American slang "chick". My partner is Swedish and people often refer to me as "hans tjej" ("his girlfriend/woman"), so I'd assume it can be used in a relationship scenario too.
Yes, that's right. En tjej can be almost any age (you can use it about children too), but en flicka is a child (it's old fashioned to refer to young women as flickor, but you can hear it sometimes, usually from older people). Min tjej is 'my girlfriend' but min flicka would be 'my daughter'.