For me Its quite simple. My brain decided it. Literally. The days I found out I’m trans, my brain would do this weird thing, where I could think about completely different stuff and suddenly my now chosen name would “fly” into my mental Field of View, like an asteroid in star wars or so. I told that name a close friend and 4 weeks later when we went to the club together I told him, that I will stick with it, because I didnt had a better idea (and it was the only name I really felt comfortable with).
It came to me in English class when I was 17. At the time I was unsure of why I became attached to the name. A few years later I realised why (and that it was a cliche, but I was attached at that point). I let my parents pick my middle name so they could have some say in naming me.