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).
(the really sucky part now is, I now have three names for myself, the only place I'm currently going by Hailey is here... and I never finished rolling out Hal, so it's kinda breaking my brain that now I want to kinda clean all that up. Domain names, usernames, display names... school is the only IRL place I managed to go by Hal, everyone else knows me by my birthname... I'm holding back on tangenting on the logistics of social name changing & of course coming out.....)