Whether you want to be a better ally, think you might be trans, or just don’t understand what this whole transgender thing is about, I highly recommend reading https://genderdysphoria.fyi/
It talks about what gender itself is, what gender incongruence feels like, the science behind how sex determination and sexual dimorphism works in humans, the diagnostic process for trans folks, and what HRT actually does and what is involved in transitioning. It’s a good read