How do I explain being trans to a nine year old?

My younger brother is nine and was asking about pride, he totally understands the lgb, some girls like girls some boys like boys and some people like both. He also kind of understands non-binary.
But the trouble came when I tried explaining trans people, my older sister is trans (and so am I though not out yet) so I used her as an example and he got completely confused as to why a boy would want to be a girl (his words) I tried explaining further but he just got frustrated. My mum says it's probably his adhd so I should find a different way to explain it but I don't know how, any ideas?
How do I explain being trans to a nine year old?
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