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’s sort of the femme version of my given name. There’s a lot of history in my family with that name that I do appreciate and value and want to feel connected to. I did a bunch of research into the names and language of where my family originated and found my new name and instantly fell in love with it. I remember sobbing after writing it down for the first time.