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).
Ayy, samesies. I love my name, and like you said, it’s a very common female name. There’s also a certain sense of power that comes from being un-deadnameable.
My government name is 99% masc, but I only hear it at work and it isn’t worth the hassle of changing it. All my mates and friends just call me “Cat”.