Genuinely transgender people do believe they were born in the wrong body and even have noticeable differences in their brains from non transgender people of the same sex so there is clearly something causing the way they feel, it's not just made up.
This is nonsense if you know anything about how the scientific method works. To actually test for these "different" brains you'd have to also test gay people, people who are gender-non-conforming but don't think they're trans, and peple in none of these categories as a control. They didn't, it was just a few trans-identifying people and it wasn't even clear what effect taking hormones could have, or how you're supposed to tell what brain markers would indicate being more like the opposite sex's brains - so it's not a reliable study.
However, what if this was true? If your brain scan showed a different/"trans" brain, should you be forced to live as trans? And why would "rea" trans peope want self-ID? Surely if they think there's a scan to show you're "really" trans, they would want that scan to be used to weed out confused teenagers who ight regret transition, or predators who were just lying to get access to women's spaces/sports etc and giving other trans people a bad name?
Why don't transactivists call for this, instead of self-ID? Self-ID is an absolute gift to predators and abusers. Why are transactivists campaigning for it if there's this scan available?
there is clearly something causing the way they feel
Yes, a thing called gender stereotypes and tomboy girls and effeminate boys, plus a lot of confused autistic kids and sexual abuse survivors, being made to feel if they don't fit into stereotypes or feel uncomfortable with their body, they must be the opposite sex. If society wasn't so sexist and homophobic they could just (actually) be who they are.