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).
I new my name 7 years before I new I was trans, I already ordered stuff on that name and my whole online persona was built around it. I even named my cat that. Even before I was out to myself I listened to that name way better than my dead name.