If I were to attempt to have a conversation with someone who isn’t gender critical, I’m not sure how I’d argue against this. They are saying that their bodies literally don’t match their brains, so that’s why they need to transition. Does anyone have any knowledge of these studies or just about this concept in general?
The question comes down to which of these two things is the determining characteristic of what it means to be female or male: the personality [as manifested by the brain], or the body?
It's fine to say that a person can have a male body, with male reproductive biology, and also feel that in his mind / sense of self, he feels more feminine than masculine, or he has interests and attributes that society deems to be 'feminine' or 'for women'. Our brains are plastic; the areas that get exercised get stronger. So it's entirely possible that a brain scan might in some cases bear out similarities between that person's brain, and findings that might be more typical among women. But you can only take the leap from there to say 'therefore that person is really a woman in a male body' if you concede to the idea that certain personality traits, interests and attributes really are solely for women, and are in fact the very defining characteristics of womanhood.
But if you believe that the defining characteristic of femaleness is having a (mammalian, sexually dimorphic) female body, and that both women and men are fully human individuals with potential for the full spectrum of human personality traits, then none of that brain scan stuff matters.
What else would a brain scan be trying to prove, other than the presence of 'female' personality traits?
I would always try to frame the discussion as 'what makes a woman a woman?' as opposed to 'what does it mean to be trans?'.
I am not trans, so I can't answer the latter question for anyone.
But I am a woman, and that qualifies me to say: "A woman is a person with a female body and any kind of personality, so it seems pointless and pretty sexist to me that anyone would be trying to categorise brains / minds / personalities as either male or female. That's regressive and offensive to women, who have fought and still are fighting to be freed from such limiting stereotypes. Why would we want to be dragged backward?"
TL;DR:
Feminists understand that the defining characteristic of femaleness is the female body, and so a woman/girl is a person with a female body and any personality or sense of self.
Genderists assert that the defining characteristic of femaleness is a personal, changeable, subjective, self-declared, unverifiable sense of female 'gender identity': essentially that a woman/girl is a person with a female personality and any (i.e. male or female) body.
Brain scans to evidence a 'female personality' only matter if you take the genderist view on what it means to be a woman.