Let me explain
Betas are normal humans
Omegas and alphas have animal like instinct they ho into heat(which is for omega) and Rut for alphas
The time their in heat js the time they are the most fertile meaning more likely to get preggy.
Pheromones
They all have cents its called pheromones. They can realise their pheromones. It's like realizing your aura and only some few ppl are able to see it.
Also female alphas have diccks
And male omegas have a uterus
So a female alphas can impregnate a omega
Population
20% of the population are omegas (majority of omegas are females btw)
while 10% are Alphas.
(majority of alphas are male)
70% being beta. Beta is a normal human being. Men can't get preggyy and women get periods. And the norm can't smell the pheromones an omega or alpha realises
If there was something I didn't mention then you'll find it out quickly by reading more omegaverse aka ao3
Hope this gives you an understanding of this
This is all