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 started as a joke (removing the D from my nickname, haha) and it stuck... So I had my new nickname and from there I tried to find a full name that could be abbreviated to my nickname, but I couldn't find one and told my cousin and my sister and both of them suggested the same name spontaneously and independently and I thought "wow I loved that name when I was young and my favourite character in my favourite show has the same name" and from there my brain kept it... It's perfect.