Here’s an explanation from Wikipedia without being forced to watch an hour-long video.

Works in the Omegaverse genre typically depict characters as possessing two sexes: a primary sex (male or female) determined by their external sexual organs, and a secondary sex that manifests during puberty, determined by their internal reproductive system.

Between Alphas and Betas, only females can carry on a pregnancy, but male Omegas are often envisaged as being able to become pregnant via a uterus connected to the rectum, and Alphas can impregnate regardless of their main gender.

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