How congenital curvature of the penis originates

The penis consists of three so-called ‘corpora cavernosa’, sponge-like columns of erectile tissue that fill with blood during an erection, making them expand and become hard.

Viewed from above, there is one corpus cavernosum stretching along each side of the penis, while the third, which also includes the glans penis, is situated below and between them.
If one corpus is shorter than the other corpora or if one side of the penis is shorter than the other side overall, it causes the penis to curve towards the shortened side when erect.

There are two types of congenital curvature of the penis: it is either already present at birth or develops during puberty (called atrophy of the corpora cavernosa), meaning that the corpora cavernosa grow to different sizes/lengths during puberty.