Whats the difference between female and woman?

“Female” is an adjective. “Woman” is a noun. 

Referring to a person as “A Female” or a group of people as “Females” is objectifying, because it reduces them to that singular characteristic. It’s kind of a subtle thing, esp. if you’re learning English as a second language. 

It functions linguistically in a very similar way to other identifying adjectives such as “black” or “trans” or “gay.”  It’s totally fine to say “a black person” or “black people” but saying “a black” or “some blacks” sounds inherently racist, because you stop describing people’s race and start defining them as nothing more than their race. 

It’s totally fine to say “a transgender person” or “transgender people” but “a transgender” or “some transgenders” sounds inherently transphobic. 

Same with “a gay person”/“gay people” vs “a gay”/“some gays” 

In addition to that, “females” as a noun is also primarily used by MRAs (”Male Rights Activists” who are misogynist) and TERFs (”Trans Exclusionary Radical Feminists” who are transmisogynist). 

So basically, for many English-speaking women, using “female” as a noun is a quick way to make us very nervous about how you perceive women.