Why is it not une médecin femme?

Bonjour Anh!

Because here "femme" is not an adjective, but a noun - "a woman doctor" - so it's "médecin" which acts like a qualifier here and therefore is placed after "femme".

I hope that's helpful!

Bonne journée !

'médecin' is masculine so 'femme' is added to qualify the gender if it's a woman but it doesn't change the noun. (If there were such a thing as 'une médecin' there'd be no need to add 'femme'.)