Well, it's up to the author tbh. Omegaverse doesn't have much of a rules which can make you experiment more about it like how alpha can turn into beta/omega, beta become omega, late bloomer and stuff.
Anyw, my guess is maybe one day it just magically appear loll? Since most omegaverse only have their second gender reveal when they reach a certain age. About the manwha, are you talking about "Love Is Fantasy"?
Well the testicles can be inside the body rather than outside and still technically be functional. As for the dick, it’s always possible that it’s a grower. Just like boys grow and then hit rapid growth in puberty, I expect the same of female alphas. A lot of omegaverse are now saying there’s 2 puberty periods. One in middle school and one in high school. The middle school one isn’t conclusive but does give a good idea of what it might be. Also it won’t be as difficult for female to have male genitals considering all males start out female in the womb and it only takes the last few months to come out male. This is also how male omegas work btw. All babies in the womb start off as females and the ovaries become testicles so it’s safe to assume the uterus just ends up staying the same. I’m sure there’s ways to tell if a baby has a uterus or not once it’s out of the womb and maybe even in the womb but it’s very expensive and might be invasive to the baby so it’s just not worth it. That’s how they can tell if a baby is intersex and has both genitals. It shows up on the scans but I don’t know how much shows up. Also the penis could very well be the clit growing into a penis much like how some transgender people get reassignment.

uhmm just a quick question in omegaverse before the female manifest to an alpha does she know she is an alpha? bcz i read in this one particular manhwa that is female have a d***, so does it grow after they manifest to an alpha or she was born with a d*** and her parents know she will be an alpha??? helppp
ps. sorry for wrong grammar