People who are attracted to others based on gender and not sex confuse me

I've had long discussions about this with my friends. Like a trans lesbian is a trans woman but I've heard people say they identify with the term lesbian as trans men. And my friend said that a trans man who's attracted to women is gay??? It's very difficult to separate gender and sex and as a dumb cis guy it's really difficult to wrap my head around

P.S. do NOT be transphobic in the comments. This is NOT an excuse to be transphobic. It is something I'm confused about that I'd like to see others opinions on.