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).
For a while when I was a kid I wondered what my name would be if I chose it myself and that was the only one that felt like it fit. Totally forgot about it until after my egg shattered and realized that it’s gender-neutral and I really like the cutesy nickname/diminutive forms