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).
I just haven't found a name I'd like to use, but my irl name is at least 98% male. Part of the problem is my name was chosen to be really similar to my non-birth-mom's name and I like that. At least there are some spellings that are used by females, even if those are uncommon, but the pronunciation doesn't change at all and I've used such as a in-game name before egg cracked. My name is relatively common from my generation. Not super-common, but also I was in a classroom once with like 5 students and three of us shared the name. Been over three years tho and still not even an inkling of an idea for alternatives.